Evidence now suggests that a bomb planted by the Islamic State (IS) group is the likely cause of a Russian airliner's crash over Egypt's Sinai peninsula, U.S. and European security sources said on November 4.
"There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," one U.S. official told CNN.
Intercepted communications played a role in the tentative conclusion that IS's Sinai affiliate planted the bomb, U.S. officials said.
If confirmed, it would be the first ever IS bomb attack on a passenger plane.
Intelligence analysts don't believe the operation was ordered by IS leaders in Raqqa, Syria. Rather, they believe it was planned and executed by IS's Sinai affiliate, which operates autonomously.
IS's Egyptian affiliate is waging a bloody insurgency in the north of the Sinai Peninsula that has killed hundreds of policemen and soldiers.
One U.S. official told the AFP news agency that downing a plane filled with Russian tourists at a time when Russia is bombing IS targets in Syria "would be something that ISIL [another acronym for Islamic State] would want to do."
The extremist group has been insistent about its involvement in the disaster since Metrojet Airbus 321-200 exploded at 31,000 feet on October 31, killing all 224 on board.
In a new statement on November 4, the affliate said the crash coincided with the anniversary of the group's pledge of allegiance to IS, and challenged skeptics to prove it was not responsible.
"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," the Sinai Province group said in an online audio recording.
British Foreign Secretary Philip Hammond said there was a "significant possibility" the crash was caused by an onboard bomb, and Britain was suspending flights to and from the Sinai resort of Sharm el-Sheikh indefinitely.
Ireland also stopped all flights to and from the resort. Hammond said Britain was advising its citizens not to go on vacation to Sharm el-Sheikh, which is visited by hundreds of thousands of Britons a year and had been a bright spot in Egypt's flagging tourist industry.
Russian and Egyptian investigators said earlier on November 4 that an internal explosion appeared to bring down the plane and the cockpit voice recorder had suffered substantial damage.
Prime Minister David Cameron's office said British aviation experts had been sent to Sharm el-Sheikh to assess security before British flights there would be allowed to resume.
Several British flights due to leave Sharm el-Sheikh for the United Kingdom were grounded, leaving hundreds of tourists stranded.
Cameron had discussed the need for tighter security at the Sharm el-Sheikh airport in a telephone conversation with Egyptian President Abdel-Fattah el-Sissi, who flew to Britain on November 4 for an official visit, Downing Street said.
The British disclosures were an embarrassment to Sissi, who had dismissed IS claims of involvement as "propaganda" and insisted that the security situation in the Sinai Peninsula was under "full control."
But the increasing likelihood that a bomb was involved prompted Egyptian authorities to replace the head of Sharm el-Sheikh airport.
Several airlines, including Lufthansa and Air France, stopped flying over Sinai after the crash, but British carriers had kept to their schedules.
Meanwhile, rescue teams in Egypt combed the Sinai desert for more remains and parts of the plane's fuselage as grief-stricken Russian families in St. Petersburg faced an agonizing wait to bury their loved ones.
Only one body has been released to a Russian family for burial so far. Relatives have identified 33 bodies and the paperwork is nearly finished on 22 of those, meaning the families should get the bodies shortly, said Igor Albin, deputy governor of St. Petersburg.
Russian and Egyptian rescue workers expanded the search area in Sinai to 40 square kilometers. The Russian state television channel Rossiya-24 reported that the plane's tail was found 5 kilometers away from the rest of the wreckage.
Forensic experts at the site are also looking for traces of explosives to confirm whether the explosion was caused by a bomb.