NAIJA MURMUR

Portraits of the 224 lives lost on a Russian passenger jet in Egypt are beginning to emerge as investigators continue to piece together clues about why the aircraft broke apart and crashed Saturday.

Olga and Yuri Shein had taken their four children on a holiday to Egypt, photos the parents posted on social media show. The couple were apparently celebrating their 10th anniversary together and their fourth year of marriage. Pictures presumably taken in Egypt show the children laughing in a pool, the couple hugging and kissing and the whole family posing playfully together.

One of the last photos the couple posted online shows Yuri holding their young daughter.

"We're flying home," his message reads.

The Russian government released the plane's passenger manifest Monday, the same day at least two planes carrying victims' bodies were expected to arrive in St. Petersburg, according to state-run news outlet RIA Novosti.

At least 25 children are among the dead, including 10-month-old Darina Gromova, whose mother took a photo of her at Pulkovo Airport in St. Petersburg waiting for their flight to Egypt. The child is standing, her hands pressed against the glass, looking out at the planes on the tarmac.

Darina Gromova's mother posted her picture before a family trip to Egypt.

The manifest shows many people with the same last name, revealing many families were aboard.

Anton Bogdanova, who had just turned 10 on October 28, was on the flight with two relatives; one of them was Anastasia Bogdanova, 22.

Alina Gaymadak, 27, posted pictures of her travels in Egypt and Israel on a social media account.

Mourners left flowers and stuffed animals and kept candles burning at makeshift memorials in St. Petersburg.

A father, Vladimir, holding his toddler, Alexander, stopped for a moment to speak with CNN near one of the memorials. "It's a horror, it's a tragedy, to lose so many children," he said. "But we can never feel the pain of their families."

Olga, a 7-year-old, held her doll and stared at the memorial. "I just wanted those children to be remembered," she said, placing her favorite toy, a blue stuffed puppy, on the mound of other toys.

People light candles and place flowers in central Saint Petersburg.

There were 217 passengers and seven crew members on Kogalymavia Flight 9268. Of the passengers, 209 were Russian, four were Ukrainian and one was Belarusian. The citizenships of three other passengers are unknown.

Russian media reported that the tragedy created a large number of orphans in Russia because many parents left their children behind to go to Sharm el-Sheikh, Egypt, a popular holiday spot.

Children's toys could be seen in piles of luggage in the middle of the Sinai desert, UK newspaper The Telegraph reported.

Relatives of passengers continue to post online, trying to grapple with their grief.

Olesya Kosorukova's mother, Natalia Rostenko, was on the flight, according to Kosorukova's social media posts, state-run Russian news agency Sputnik reported.

"Mother, come back," she wrote. "I will always be waiting for you."

Russian officials on Monday reiterated that it is too early to know what caused a passenger jet to break apart and crash in Egypt this week, challenging an earlier comment from an airline executive who ruled out technical or mechanical problems.

Kogalymavia Flight 9268 broke into pieces before it hit the ground in a remote area of Egypt's restive Sinai Peninsula on Saturday, killing all 224 people on board.

Kogalymavia airline official Alexander Smirnov told reporters in Moscow that the protection systems on the plane would have prevented it from crashing, even if there were major errors in the pilot's control equipment.

21 photos: Russian plane crashes in Egypt

"Therefore the only reason that could explain the plane's breaking up in the midair can be a certain impact, purely mechanical (and/or) physical impact on (the) flying vessel," he said.

The vague comment made translation difficult, with some interpreting that the executive said an "external influence" caused the crash.

The head of the Russian Federal Air Transport Agency, however, quickly pushed back.

"It is completely premature to speak about the reasons of this, as there are not grounds. And I'd like to call on the aviation community to refrain from any premature conclusions," Transport Agency chief Alexander Neradko told the state-run Russian news agency Sputniknews.com.

First victims' bodies arrive in Russia 02:14

Experts agree it is too early to know for certain what caused the plane to break up at the start of what could be a lengthy investigation. CNN aviation correspondent Richard Quest suggested that the Kogalymavia official could have meant something abnormal and out of the ordinary had occurred.

However, Smirnov said that while the plane's flight and voice data recorders had been recovered, they had not been read or decoded. Officials have played down an apparent claim by Islamic militants in Sinai that they brought down the Airbus A321-200, saying technical failure is the most likely reason for the crash.

Here's where things stand:

THE FLIGHT

Flight 9268 was on its way from the Egyptian resort of Sharm el-Sheikh to St. Petersburg early Saturday when it dropped off radar about 23 minutes into the flight, Egyptian officials say.

Air traffic controllers apparently didn't receive any distress calls from the pilots. "There was nothing abnormal before the plane crash," Egyptian Civil Aviation Minister Hossam Kamel said Saturday. "It suddenly disappeared from the radar."

CNN's Quest said it was "unusual" for an aircraft to go down roughly 20 minutes into a flight.

Richard Quest: Russian plane crashed at what should have been flight's safest point

Russian plane crash: Quest finds it mysterious

"At this point, a plane is on autopilot. It's reaching its initial cruising altitude, and there is little that can or should go wrong," he wrote in an analysis.

But the website Flightradar24, which tracks aircraft around the world, said it had received data from the Russian plane suggesting sharp changes in altitude and a dramatic decrease in ground speed before the signal was lost.

"It's disturbing to me. It indicates to me that something occurred possibly in the way of aerodynamic stall. I mean, an airplane just cannot fly at those lower speeds," said CNN aviation analyst Les Abend, although he cautioned that the Flightradar information was very preliminary.

Analysis: Plane crashed at what should have been flight's safest point

THE CRASH

"Disintegration of the fuselage took place in the air, and the fragments are scattered around a large area" covering about 20 square kilometers (8 square miles), Viktor Sorochenko, executive director of Russia's Interstate Aviation Committee, told reporters Sunday.

Learning that the plane broke into pieces while in the air helps reduce the list of possible causes of the crash, but there are still plenty of scenarios, said CNN aviation analyst Peter Goelz.

Russian jet crash investigation -- the key question

"It narrows it down a little bit, but there are a number of issues that could have affected this plane," said Goelz, a former managing director of the U.S. National Transportation Safety Board, or NTSB. "And terrorism has not been ruled out."

He suggested the disaster could have resulted from "some sort of catastrophic failure, perhaps caused by an earlier maintenance problem. It could have been a center fuel tank that might have exploded."

Former NTSB investigator Alan Diehl told CNN he believes the "final destruction" of the plane could have been from "aerodynamic forces or some other type of G-forces."

Investigators are expected to get a clearer idea of what happened to the aircraft from its flight data and cockpit voice recorders -- devices commonly known as black boxes -- both of which have been recovered, according to authorities.

THE PLANE

The Airbus A321-200 operated by the airline Kogalymavia passed a routine inspection before takeoff, Egyptian Airports Co. chief Adel Al-Mahjoob said Saturday.

Report: Russian plane's condition concerned co-pilot 03:29

According to the Aviation Safety Network, which tracks aircraft incidents, the same plane's tail struck a runway while landing in Cairo in 2001 and required repair. At the time, the aircraft was registered to the Lebanese carrier Middle East Airlines, registration records show.

Kogalymavia's Andrei Averyanov confirmed to reporters Monday that the plane had been damaged in 2001 but said it had most recently been thoroughly checked for cracks in 2013. Not enough time had passed for major cracks to develop to a critical size since then, he said.

The ex-wife of the plane's copilot, Sergei Trukhachev, said over the weekend that he had told his daughter he was concerned about the condition of the plane. "Our daughter had a telephone chat with him just before the flight," Natalya Trukhacheva told Russia's state-run NTV. "He complained before the flight that one could wish for better technical condition of the plane."

Russia mourns those lost in Egypt plane crash 01:47

But a Kogalymavia said such reports were irresponsible and that there was no record of the pilot or crew making any complaints.

Executive Smirnov said that he had personally flown the plane in recent months and that it was "pristine."

The A321-200 was built in 1997, and Kogalymavia, which is also known as Metrojet, had been operating it since 2012, Airbus said in a statement. The aircraft had clocked around 56,000 flight hours over the course of nearly 21,000 flights, the plane maker said.

The Irish Aviation Authority said that the plane was registered in Ireland to Wilmington Trust SP Services (Dublin) Ltd, which leased the aircraft to Kogalymavia.

"As an Irish registered aircraft, in April/May 2015, the Irish Aviation Authority (IAA) conducted an annual review of the aircraft certifications in support of its annual Certificate of Airworthiness renewal process and all certifications were satisfactory at that point in time," it said.

THE VICTIMS

There were 217 passengers and seven crew members on board Flight 9268. Of the passengers, 209 were Russian, four were Ukrainian and one was Belarussian. The citizenships of three other passengers are unknown.

Egyptian medics have retrieved 175 bodies from the scene, a medical source in Sinai told CNN on Monday.

Most of the bodies retrieved are intact, the source said.

About 60% to 70% percent of the bodies have an intact core, with some having dismembered limbs, the source added.

"There are no major burns," the source told CNN. The source believes the rest of the bodies on site now are not intact, and this why they would take longer to retrieve.

The photo of 10-month-old Darina Gromova posted by the baby's mother before a family trip to Egypt.

Twenty-five children were among the victims, including 10-month old Darina Gromova, who is shown in a photo posted on social media by her mother on October 15 at the start of the family's trip to Egypt. In the photo, Darina is looking out a window at planes on the tarmac of Pulkovo Airport in St. Petersburg.

Russian media reported that the disaster created a large number of orphans in Russia, as a lot of parents left their young children with relatives while they took vacations in Sharm el-Sheikh.

Early Monday, a Russian plane carrying the remains of 144 of the crash victims landed in St. Petersburg, Russia's state-run news agency RIA Novosti reported.

Another plane bringing more bodies is expected to depart Egypt later Monday.

Russian President Vladimir Putin met with his country's transport minister Monday to discuss the investigation and again expressed his condolences to the victims' families.

"This is a terrible tragedy and we are most certainly with you at this time with all our hearts and souls," Putin said, according to a Kremlin transcript of their meeting.

"I want to thank the people of St. Petersburg for the way they have responded. The whole country has seen this, everyone in Russia, and I want to thank you for your words of sympathy and condolence. In such tragic hours, it is certainly very important to feel the support of those close to you and know you have the entire country's sympathy over this terrible disaster."

THE INVESTIGATION

Egyptian President Abdel Fattah el-Sisi on Sunday urged the public not to jump to conclusions. "These are complicated matters that require advanced technologies and wide investigations that might go on for months," he said.

Aviation official: Russian jet disintegrated in mid-air 03:16

Al-Sisi has promised Putin that he will allow "the broadest possible participation of Russian experts in the investigation," according to the Kremlin, and Russian officials have joined their Egyptian counterparts at the crash scene. Putin has also ordered Russian Prime Minister Dmitry Medvedev to open an investigation into the crash, the Kremlin said.

Aviation investigators from France and Germany, the countries where the plane was manufactured, are also taking part in the inquiry.

Egyptian officials said Saturday that the two "black boxes" were being transported to Cairo for analysis.

The flight data recorder stores a vast array of information about the flight, such as air speed, altitude, engine performance and wing positions. The cockpit voice recorder captures sounds on the flight deck that can include conversations between the pilots and warning noises from the aircraft.

Russia's state-run official TASS news agency reported Monday that the "black boxes" had been inspected by top Russian officials and were said to be in a good condition.

Egypt's Ministry of Civil Aviation later released a photo of one of the boxes.

Photo released by Egypt's Ministry of Civil Aviation.

THE REGION

In recent years, the Sinai Peninsula has been a battleground between ISIS-affiliated militants and Egyptian security forces. The vicious conflict has killed hundreds of people.

The militants appeared to claim responsibility for bringing down the Russian passenger jet in a statement posted online Saturday, but officials in Egypt and Russia disputed it.

Russian jet crash investigation -- the key question

Mahjoob, the airport official, said there was no evidence of a terrorist attack. And Russian Transport Minister Maxim Sokolov said the claim that terrorists brought down the plane with an anti-aircraft missile "cannot be considered reliable," according to RIA Novosti.

The Egyptian military said militants in Sinai have shoulder-fired anti-aircraft weapons that shoot only as high as 14,000 feet, far short of the more than 30,000 feet at which Flight 9268 was flying when it dropped off radar.

To reach such an altitude would require missiles using special launch pads and radar systems operated by engineers, the military said.

Kremlin spokesman Dimitry Peskov refused to discount terrorism, telling CNN's Matthew Chance on Monday that "only (the) investigation can rule out something."

Kogalymavia executives also said Monday that it was too early in the investigation to speculate or draw any conclusions. But Smirnov referred to purported footage of the crash posted by militants, saying: "Those images you have seen on the Internet, I think they are fake."

CNN's Sara Sirgany, Mariano Castillo, Catherine Shoichet, Nic Robertson, Alla Eschenko, Alexander Felton, Arwa Damon, Nancy Leung and Radina Gigova contributed to this report.