President Obama said Thursday it is possible a bomb was placed on a Russian passenger jet that crashed in Egypt.
"I think there is a possibility that there was a bomb on board, and we're taking that very seriously," Obama said in an interview with Seattle radio station KIRO.
But the president cautioned that the U.S. has made no official determination on the cause of the crash, which happened last weekend in the Sinai Peninsula.
American officials are working to determine why the plane broke apart in mid-air.
The Airbus A321 was at 33,000 feet when it broke apart mid-flight on Saturday, scattering debris over a wide area and killing all 224 passengers and crew on board.
The Islamic State in Iraq and Syria (ISIS) has claimed responsibility for the downed plane.
“The fighters of the Islamic State were able to down a Russian plane over Sinai province that was carrying over 220 Russian crusaders,” the group said in a statement posted online on Saturday. “They were all killed, thanks be to God.”
Officials dismissed the notion that the plane was shot down, saying it was out of the range of anti-aircraft weaponry, but are open to the possibility that a bomb was placed on board before take-off.
They have also left open the possibility that the crash was caused by a mechanical failure.
Other nations have been more willing to point to terrorism. British Prime Minister David Cameron said on Thursday that terrorism is "more likely than not" the reason for the plane's crash.