An “engine explosion” is thought to be the most likely cause of the Russian plane crash in Sinai on Saturday which killed 224 people, according to Egyptian and Russian media reports.

Investigators have been carrying out preliminary analyses of the Metrojet Airbus 321’s two “black box” flight recorders since Tuesday morning, though damage to the equipment has slowed their progress.

But while an initial assessment of the evidence on the cockpit voice recorder seemed to show noises “uncharacteristic of a standard flights” just before the plane disappeared off radar screens, a first look at the other recorder has thrown up an explosion of the engine as the main lead.

That’s according to a source close to the investigation, the Egyptian Al-Masry al-Youm newspaper reported. The source also said that the possibility of a catastrophic blast on the engine was first thrown up by an analysis of debris at the crash site.

In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015, showing Metrojet Airbus A321-200 flight 7K9268 flight recorder on display at an undisclosed location in Egypt

While reports from investigation sources to Egyptian and Russian media have been common, official statements say that a proper analysis of both data recorders will take between one and four weeks.

Until then, conflicting reports as to the cause of the crash continue to emerge.

Data released by the flight-tracking website FlightRadar24 have given further insight into the final seconds of the St Petersburg-bound flight from Sharm el-Sheikh.

Russian passenger plane crashes in Egypt Show all 20 1 /20 Russian passenger plane crashes in Egypt Russian passenger plane crashes in Egypt Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula Russian passenger plane crashes in Egypt Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula Russian passenger plane crashes in Egypt In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015, showing Metrojet Airbus A321-200 flight 7K9268 flight recorder on display at an undisclosed location in Egypt Russian passenger plane crashes in Egypt Mourners lay flowers at Pulkovo International Airport outside St. Petersburg. Russia on 1 November mourned its biggest ever air disaster after a passenger jet full of Russian tourists crashed in Egypt's Sinai, killing all 224 people on board. Flags were at half mast on the parliament building, in the Kremlin, and on other official buildings in honour of the victims, most of whom were from Russia's second-largest city of Saint Petersburg Russian passenger plane crashes in Egypt People pay their respects at the entrance of Pulkovo airport outside St. Petersburg, during a day of national mourning for the plane crash victims Russian passenger plane crashes in Egypt Debris from the plane crash in Egypt Russian passenger plane crashes in Egypt A piece of an engine of Russian MetroJet Airbus A321 at the site of the crash in Sinai, Russian passenger plane crashes in Egypt The crash site debris Flight 7K9268 crashed in the Sinai peninsula, in all probability killing every one of the 224 people on board AFP/Getty Russian passenger plane crashes in Egypt The crash site debris Debris lies strewn across the sand at the crash site EPA Russian passenger plane crashes in Egypt Relatives in St Petersburg Relatives react after a Russian airliner with 217 passengers and seven crew aboard crashed, as people gather at the Kogalymaviaís information desk at Pulkovo airport in St Petersburg on 31 October AP Russian passenger plane crashes in Egypt Relatives in St Petersburg A relative of a passenger of MetroJet Airbus A321 at Pulkovo II international airport in St Petersburg, Russia, 31 October 2015. EPA Russian passenger plane crashes in Egypt The plane's journey The plane's last recorded radar position above the northern Sinai peninsula Flightradar24 Russian passenger plane crashes in Egypt Where it crashed A satellite view from Google Maps of the rough area where the plane crashed, in the mountainous Hassana region of the Sinai peninsula. Google Maps Russian passenger plane crashes in Egypt The plane The Metrojet's Airbus A-321 with registration number EI-ETJ that crashed in Egypt's Sinai peninsula REUTERS/Kim Philipp Piskol Russian passenger plane crashes in Egypt The plane The crashed Airbus A321 at Domodedovo international airport, outside Moscow,, on 20 October Russian passenger plane crashes in Egypt Relatives at in St Petersburg A relative of a passenger on MetroJet Airbus A321 at Pulkovo II international airport in St Petersburg EPA Russian passenger plane crashes in Egypt Relatives at in St Petersburg Relatives of passengers of MetroJet Airbus A321 at the Crown Plaza hotel in St Petersburg EPA Russian passenger plane crashes in Egypt Bodies being repatriated An Egyptian soldier prays as emergency workers prepare to unload bodies of victims from a police helicopter to ambulances at Kabrit military airport on 31 October. AP Russian passenger plane crashes in Egypt Bodies being repatriated Ambulances line up as emergency workers unload bodies at Kabrit military airport, 20 miles north of Suez, on Saturday AP Russian passenger plane crashes in Egypt Bodies being repatriated Egyptian paramedics load the corpses of victims into a military plane at Kabrit military air base by the Suez Canal on October 31, 2015 AFP/Getty Images

It said the plane reached its cruising altitude before abruptly slowing down, after which it started plummeting to the ground at 300mph. It dropped from 31,000ft to 26,000ft in a matter of moments, after which its transponders stopped transmitting data.

Egypt: FIRST SHOTS of the crashed Russian plane in Sinai

Also today, the Isis militant group released a new audio message claiming responsibility for bringing the plane down – but failed to provide any evidence to back this up.

The message, released by Isis’s branch in the Sinai Peninsula, instead challenged the Egyptian and Russian authorities to “prove we didn’t do it”.

While officials have not conclusively ruled out a terrorist incident, experts said it was unusual for Isis to claim an attack without providing some form of evidence to show it did so.