President Obama said Thursday the U.S. is heavily weighing the possibility that a bomb brought down a Russian passenger jet in Egypt's Sinai Peninsula on Saturday.
The president told Seattle radio station KIRO the U.S. is "very seriously" considering the possible new development.
A U.S. intelligence source confirmed to Fox News earlier in the day that intelligence agencies indeed have preliminary evidence, including intercepts, suggesting a bomb brought down Metrojet Flight 9268.
Still, a source tells Fox News the intelligence is not conclusive. Other than aviation fuel, no bomb residue has been found on the wreckage or victims, according to this source, who has been briefed on the latest findings.
The flight’s passenger manifest is being run through watch lists and terror databases in the U.S. to identify suspect individuals.
White House spokesman Josh Earnest has said earlier in the day that the U.S. can't rule out the possibility of terrorism in the crash that killed all 224 people onboard.
Earnest said the U.S. hadn't officially determined what caused the crash.
British Prime Minister David Cameron said he has grounded all British flights to and from the Sinai Peninsula because of "intelligence and information" that points to a bomb as the probable cause of Saturday's crash.
Cameron's Foreign Secretary Philip Hammond said that there was a "significant possibility" that an affiliate of the Islamic State terror group brought down a Russian passenger plane in Egypt's Sinai Peninsula Saturday, killing 224 people.
A spokesperson confirms to Fox News that British Prime Minister David Cameron called Russian President Vladimir Putin to discuss the crash. Egyptian President Abdel-Fattah el-Sisi, who visited Prime Minister Cameron in London Thursday, called the ISIS claim "propaganda."
The jet that crashed Saturday was traveling from Sharm el-Sheikh to St. Petersburg when it crashed 23 minutes after takeoff. The overwhelming majority of the dead were Russian vacationers who were returning home.
On Wednesday, ISIS released a fresh claim of responsibility for downing the jet on a social media account well known to intelligence circles, an intelligence source told Fox News. The message said the onus was not on ISIS to explain how they brought down the plane, but it was up to the West to figure it out.
"We've looked at the whole information picture, including that claim," Hammond told Sky News, referring to a claim of responsibility by an ISIS affiliate in Sinai hours after the crash, "but of course lots of other bits of information as well, and concluded that there is a significant possibility."
"We don't know for certain that it was a terrorist bomb ... (but it's a) strong possibility," Cameron said at his London office at 10 Downing St.
A U.S. official briefed on the investigation told the Associated Press that there had been no formal judgment rendered by the CIA or other intelligence agencies, and that forensic evidence from the blast site, including the airplane's black box, was still being analyzed.
The official added that intelligence analysts don't believe the operation was ordered by Islamic State leaders in Raqqa, Syria. Rather, they believe that if it was a bomb, it was planned and executed by the Islamic State's affiliate in the Sinai, which operates autonomously.
A spokesman for the Russian president insisted that aviation investigators were working on all possible theories as to why the Airbus A320-200 crashed and said naming just one possibility was mere speculation.
"One cannot rule out a single theory, but at this point there are no reasons to voice just one theory as reliable -- only investigators can do that," Dmitry Peskov told reporters in Moscow.
Egyptian Antiquities Minister Mamdouh Eldamaty also rejected the U.S. and British allegations outright.
"(The crash) not a terror act. It was an accident," he declared Thursday, speaking at the ancient city of Luxor as authorities opened three tombs to the public for the first time in an effort to encourage tourism. "(It's) very sad what happened, but we have to wait for the result of the investigation."
Egypt's presidential spokesman, Alaa Youssef, also said Egyptian authorities wish the U.S. and Britain had "waited for the result of the ongoing investigation."
Meanwhile, the British government prepared for a massive effort to repatriate an estimated 20,000 British tourists who have been stranded after all flights to and from the Sinai region were suspended indefinitely Wednesday. Sky reported that 19 flights were due to leave British airports for Sharm el-Sheikh Thursday. All have been canceled, but Hammond said special flights from Sharm el-Sheikh to Britain were expected to begin Friday.
Several airlines, including Lufthansa and Air France, stopped flying over Sinai after the crash, but British carriers had kept to their schedules. Almost 1 million Britons visit Egypt each year.
Early Thursday, the Dutch government announced it would also cancel flights to Sharm el-Sheikh in the wake of the crash.
The head of Sharm el-Sheikh's airport was replaced Wednesday. Adel Mahgoub, chairman of the state company that runs Egypt's civilian airports, says airport chief Abdel-Wahab Ali has been "promoted" to become his assistant. He said the move had nothing to do with media skepticism surrounding the airport's security and that Ali is being replaced by Emad el-Balasi, a pilot.
British travelers told Sky News of lax security procedures at the airport, with one traveler writing, "The person manning the scanning machine was playing Candy Crush on his phone. Once we were through, my exact words to my husband were that I hoped nobody on our flight has a bomb today."
Another wrote, "On our last visit ... the security official on the baggage scanner was too busy sleeping as opposed to looking at the bags going through the scanner. Since then we have decided never to return."
Sarah Cotterill, who was to fly to Gatwick on easyJet Wednesday, says she was about to "board the plane" home when the suspension was announced.
She told BBC that "after spending about three hours at the airport we've been bussed back to our hotel, and that's where we are at the moment."
Paul Modley, a 49-year-old Londoner who has travelled to Sharm el-Sheikh seven times in the last nine years, says he worries for Egypt's tourism.
Modley, due to fly home on Saturday, says he is "really worried for the Egyptian people because — particularly in the Red Sea resorts — they are so dependent on tourism."
Fox News' Jennifer Griffin, Catherine Herridge, Lucas Tomlinson and the Associated Press contributed to this report.