The British government and anonymous American officials have said there's reason to believe a bomb brought down the Russian airliner that crashed in Egypt's Sinai peninsula over the weekend.
Hours earlier, the Islamic State released a pair of propaganda messages again taking responsibility for the disaster. The Russian Airbus A321 crashed en route to St. Petersburg from the resort town of Sharm el-Sheikh on Saturday, killing all 224 people on board. Initially, US officials questioned whether the group had the capability to take down a large passenger aircraft, but in videos released Wednesday, the militants challenged the world to prove they didn't do it.
"Take the crashed plane and search it, take the black box and analyze it," said a representative of the Islamic State's Sinai-based affiliate in an audio message titled "We Downed It, So Die in Your Rage." "Tell us what you found in your investigation, show us your expertise and prove, if you can, that it wasn't us who took the plane down or how it fell."
The message explained that the attack was designed to be "[a] response to Russian airstrikes that killed hundreds of Muslims on Syrian land," but did not offer any details about the alleged attack. "[We are] under no obligation to reveal the method by which we took [the plane] down," the message said. "We were the ones, with God's grace, who made the plane fall and we will reveal how we took it down when and how we see fit."
A separate video, released by the Islamic State's affiliate in Nineveh, Iraq, also claimed responsibility. That video featured Russian-speaking members of the Islamic State, and reiterated that the attack was revenge for Russian airstrikes against Islamic State positions in Syria. "I would like to congratulate our brothers from the Sinai peninsula for the downing of that cursed plane of the Kafirs (unbelievers) from Russia," a man says in Russian in the Ninevah video.
"I would like to address you Putin, you pig," the man says, raising a knife. "I would like to tell you that when you send your planes here, your people and your equipment to destroy our state and us, that you were deeply mistaken that you would not regret this and that your people would not regret this, and this plane is confirmation of that."
The actual cause of the crash is still a matter of intense speculation. What is known is that the plane left the resort town of Sharm el-Sheikh at around 6am local time on Saturday, then disappeared from radar screens 20 minutes late.
Officials with Metrojet, the company that operated the plane, have claimed that there is no evidence of pilot error or technical failure. "The only explainable cause is physical impact on the aircraft," Alexander Smirnov, deputy general director of Metrojet, said.
Egyptian authorities are leading the investigation, and Egyptian President Abdel Fatah el-Sissi said on Wednesday that claims of terrorist involvement are "propaganda." But on Monday, James Clapper, the US director of national intelligence, was more cautious. "We don't have any direct evidence of any terrorist involvement yet," he said. "I wouldn't rule it out."
There's widespread consensus that the Islamic State does not possess the capabilities to shoot down the plane from its cruising altitude of 30,000 feet. The group is known to possess some man-portable air-defense systems (MANPADS), but the weapons can only reach targets at 10,000 feet or lower.
On Wednesday, the office of UK Prime Minister David Cameron emailed a statement to reporters suggesting that a bomb may have brought down the plane. British authorities are not formally involved in the crash probe, and did not release details to explain why they have reason to believe it was a bombing.
"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed," Cameron's office said. "But as more information has come to light, we have become concerned that the plane may well have been brought down by an explosive device."
The UK announced on Wednesday it would suspend flights between Sharm el-Sheikh and Britain as "a precautionary measure."
Later on Wednesday afternoon, CNN quoted a "US official familiar with the matter" as saying the plane was most likely taken down by a bomb. "There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," the unnamed official said, stressing that the US intelligence community has not made a "formal conclusion" about the cause of the crash. The Associated Press and other outlets later quoted anonymous US officials as saying the plane was likely bombed.
The local press in Egypt is reporting that the "black box" flight recorder recovered from the crash site contains some evidence of foul play, but details are sparse. "This is all speculation," said Egypt's Civil Aviation Minister Hossam Kemal. "There is nothing definitive until the investigation commission completes its probe."
Editor's Note: This story has been updated since it was initially published to include new details about the suspected cause of the crash.
Follow Avi Asher-Schapiro on Twitter: @AASchapiro