WASHINGTON -- The United States said it can't rule out the possibility of terrorism in the crash of a Russian plane in Egypt's Sinai Peninsula that killed 224 people onboard
CBS News national security correspondent David Martin reports that U.S. intel now regards terrorism as "the leading theory."
President Obama told KIRO Radio's Dave Ross on Thursday, "I think there is a possibility that there was a bomb on board. And we're taking that very seriously."
Mr. Obama continued, "We know that the procedures we have here in the United States are different than some of the procedures that existed for outbound and inbound flights there. We're going to spend a lot of time making sure that our own investigators and our own intelligence community figures out what's going on before we make any definitive pronouncements."
White House spokesman Josh Earnest said previously on Thursday the U.S. hadn't determined what brought down the Metrojet flight last Saturday.
Earnest said he can't discuss what U.S. intelligence officials have learned so far.
He also said the administration is reviewing what steps it can take to enhance security on commercial flights to the United States from certain countries.
Islamic State of Iraq and Syria (ISIS) militants are taking responsibility for the crash -- as a response to Moscow's recent military intervention in Syria against the extremist group. But ISIS hasn't provided any evidence to support that claim.
A senior U.S. State Department official said Secretary of State John Kerry has discussed the crash of the Russian airliner with his Egyptian counterpart.
The official said Kerry told Egyptian Foreign Minister Sameh Shoukry in a telephone call Thursday that although media outlets have reported on suspected causes of the crash, the U.S. government has yet to make an official determination. Kerry also expressed condolences for the crash.
A flight tracking website shows the Russian plane plunging to the ground at 300 miles an hour.
Aviation consultant Denny Kelly said, if there was a bomb on the Airbus A321, the wreckage and bodies will provide the evidence.
"There are going to be certain types of markings on the body, and there's going to be certain types of things, certain markings inside the airplane," he said, adding that such evidence "can readily be discovered at the scene if the people know what they're doing."
British airlines Monarch and easyJet said they will fly from Egypt's Sinai Peninsula Friday to bring stranded British tourists back.
EasyJet said it will run nine flights from the Red Sea resort to London airports and one to Milan. Monarch will run two scheduled flights and three additional flights. Neither airline is operating outbound passenger flights from the U.K. to Sharm el-Sheikh.
The U.K. grounded all flights to and from Egypt's Sinai Peninsula Wednesday, saying there was a "significant possibility" the Russian airliner that crashed Saturday was downed by a bomb. The government has said it could take a week to 10 days to fly out the 20,000 Britons currently in Sinai.
The Egyptian minister for civil aviation insisted Thursday that its airport security is up to international standards, and said there was no evidence yet to support the theory that a bomb was planted on the Metrojet plane that came apart over Egypt's Sinai Peninsula.
"The investigation team does not have yet any evidence or data confirming this hypothesis," Hossam Kamal said.
CBS News correspondent Allen Pizzey said new video, reportedly taken shortly after the crash, shows smoke rising from the wreckage as Egyptian ambulance workers move around the scene.
Officially, U.S. intelligence agencies have declined to speculate on the cause in order not to prejudice the ongoing investigation.
Off the record, however, intel sources have told CBS News they're leaning toward the bomb theory, based at least in part on "chatter" between ISIS suspects on social media.
Russian investigators said Thursday they were wrapping up the search of the Metrojet crash area in Sinai.
Teams analyzing the flight data and cockpit voice recorders -- the latter of which reportedly sustained damage -- could have as much as a month of work left, however.
They're urging patience and saying nothing about bombs, or anything else.