Evidence suggests an explosion -- and not a mechanical malfunction -- brought down the Russian passenger jet over Egypt, two sources told Fox News Wednesday.

One congressional source put the chance of an explosion at 80 percent. That source says an ISIS device is under strong consideration, but that investigators have not reached a conclusion.

An intelligence source confirmed to Fox News late Wednesday that U.S. intelligence agencies have preliminary evidence, including intercepts, suggesting a bomb brought down the Russian plane.

The flight’s passenger manifest is being run through watch lists and terror databases in the U.S. to identify suspect individuals, Fox News confirms.

ISIS released a new message claiming responsibility for downing the jet, an intelligence source says. Like the first claim, this one comes from a senior ISIS social media account that is well known in intelligence circles. The new message claims the terror group was responsible, and that the onus is not on ISIS fighters to explain how they did it, but up to the West to figure it out.

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Also Wednesday, the UK Prime Minister’s Office announced the jet “may well have been brought down by an explosive device."

A spokesman said British investigators were headed to Egypt's Sharm el-Sheikh Airport to analyze the security there, and delayed all flights to the UK from that airport "as a precautionary measure."

If a bomb were involved in the crash, it’s highly unlikely the explosive came in contact with U.S. carriers because they are not permitted to fly out of Sharm el-Sheikh Airport, White House Press Secretary John Earnest told reporters during a Wednesday press conference. There are also no direct flights from that airport to the U.S.

Earnest said the U.S. was cooperating with teams from Egypt, which have taken the lead in the investigation, and with teams from Russia.

Cockpit recordings from the plane revealed unusual sounds the moment it went off the radar, Russian media reports say.

An unnamed source told Russian news agency Interfax that “sounds uncharacteristic of routine flight were recorded preceding the moment that the aircraft disappeared from radar screens,” according to The Guardian.

The news agency said it had viewed a transcript of the cockpit recordings recovered from black boxes being examined by Egyptian officials. The crash killed all 224 people on board.

The source said a situation had developed on the plane “suddenly and unexpectedly.” Investigators have yet to release a report on the crash and pilots spoke to air traffic controllers in a routine exchange four minutes before the plane went down, The Guardian reports.

Families so far have identified the bodies of 33 victims killed in the crash, Igor Albin, deputy governor of St. Petersburg, told The Associated Press. Most of them were vacationing from Russia's St. Petersburg.

Emergency Situations Minister Vladimir Puchkov said Wednesday that rescue teams in Egypt have expanded the search area to 15 square miles.

Russian officials have refrained from announcing the cause of the crash, citing the ongoing investigation.

ISIS has released a second claim of responsibility for the downing of the Russian jet, but intelligence officials have rejected those claims as propaganda.

FoxNews.com’s Catherine Herridge and The Associated Press contributed to this report.