WASHINGTON -- Security screening is being tightened, and flights are being cancelled as investigators look for evidence of a bomb in the downing of a Russian jet over Egypt last weekend which killed all 224 passengers on board.

The cause of the explosion at 31,000 feet is not definite. But that's little comfort to travelers heading into the holidays -- the busiest time for airlines.

Sources say the sound of an explosion can be heard on the cockpit voice recorder -- although that still does not conclusively point to a bomb. But with U.S. intelligence closer to a consensus that this was a terrorist act, security is being stepped up at overseas airports with direct service to the U.S.

Reuters/Mohamed Abd El Ghany

Everything brought aboard a jet will be subject to extra screening -- from checked and carry-on luggage to possibly even catering and other items loaded on planes. Homeland Security will also conduct an "airport assessment" of passenger and baggage screening procedures at about 10 airports across the Middle East and North Africa.

Friday, Russian President Vladimir Putin ordered the suspension of all flights between his country and Egypt. At an anti-terrorism meeting in Moscow, Russian officials revealed that they have taken samples from the crash site that will be tested for traces of explosives.

That forensic analysis could help confirm evidence that is pointing to the possibility of a bomb.

The U.S. has satellite images that show a series of heat flashes at the time the plane went down -- although it is not clear whether the flashes were from a bomb, explosion, or mechanical failure. And U.S. and British intelligence have intercepted chatter from ISIS talking about the crash, but has not verified its credibility.

Former TSA Administrator, John Pistole, says if ISIS did get a bomb on board, it's a game changer in the battle against terrorism.

"If they are now active, then that does raise significant concerns because of their capabilities. You are not just talking about a singular airport here. You are talking about perhaps dozens of airports that they might have access to infiltrate one of their operatives or to bribe an unwitting airport employee," Pistole said.

The Russian government remains tight lipped about the ongoing investigation. But we've learned Friday night that it has requested support from the FBI, most likely for its bomb analysis expertise.