The Islamic State on Wednesday insisted it was behind Saturday’s downing of a Russian passenger jet in the Sinai Peninsula, but said it was not obligated to elaborate on how the alleged attack was carried out.

All 224 people on board the Airbus A321 flight bound for Saint Petersburg, which crashed shortly after taking off from a south Sinai resort, were killed.

Investigators continued on Wednesday analyzing data from the black boxes of the plane, a charter flight operated by Russian airline Kogalymavia, which went down 23 minutes after takeoff from Sharm el-Sheikh.

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Authorities have warned that the probe could last several weeks or months, depending on the conditions of the recordings in the black boxes.

IS provided no details in its claim of responsibility for the crash. It has met with skepticism from Russia and Egypt, but officials say they cannot rule out any possibilities.

In another statement on Wednesday, IS insisted it had brought down the plane, challenging skeptics to prove otherwise.

“We are under no obligation to explain how it came down,” the group said in the audio statement posted on social media sites.

“Prove that we didn’t bring it down, and how it came down. We will detail how it came down at the time of our choosing.”

British Prime Minister David Cameron’s office said he and Egyptian President Abdel Fattah el-Sissi agreed in a phone call Tuesday on the need for tight security at the Sharm el-Sheikh airport.

Cameron and Sisi “agreed it was important not to pre-judge the investigation,” the British statement said.

They “noted that there was still uncertainty about the cause of the crash and agreed it would be prudent to ensure the tightest possible security arrangements at Sharm el-Sheikh airport as a precautionary measure.”

Sissi, who departed for London on Wednesday, has downplayed the IS claim of responsibility for the plane crash as “propaganda” but said a full investigation will take time.

US Director of National Intelligence James Clapper also said this week it was “unlikely” IS was involved, but he did not rule out the possibility.

The airline itself has ruled out technical fault or human error, drawing fire from the head of Russia’s aviation authority. Alexander Neradko said the airline’s conclusions were “premature and not based on any real facts.”

Experts say the fact that debris and bodies were strewn over a wide area points to a mid-air disintegration of the aircraft.

That leaves two possibilities — a technical fault that caused the plane to disintegrate or an explosion caused by a bomb smuggled on board, according to experts.

US officials told CNN and other US television networks that a military satellite had detected a heat flash at the time of the crash, which could point to a catastrophic event during flight, possibly a bomb explosion.

Search operations had been extended to a radius of about 40 kilometers (25 miles). President Vladimir Putin has described the crash — Russia’s worst-ever air disaster — as a “huge tragedy.”

Relatives of the victims have begun the process of identifying the bodies after two planes delivered the remains of many to Saint Petersburg.

Saint Petersburg deputy governor Igor Albin told reporters that 33 crash victims had already been identified.

Family members provided DNA samples at a crisis center set up near Saint Petersburg’s Pulkovo airport, now the site of an impromptu memorial where people have brought flowers and toys to commemorate the victims, many of them children.

IS has deployed shoulder-fired anti-aircraft missiles in the past but they are not known to possess weapons that could bring down an airliner flying at high altitude.

The militants have killed hundreds of policemen and soldiers since the military overthrew Islamist president Mohamed Morsi in 2013.

They say their attacks have been in retaliation for an ensuing police crackdown in which hundreds of Morsi supporters have been killed and thousands, including the ousted president, jailed.

In an interview with the BBC ahead of his trip to Britain, Sissi insisted the Sinai “is under our full control.”