After Metrojet officials said Monday that the “only possible explanation” for the high-altitude break-up Saturday of a Russian jetliner over the Sinai Peninsula “could be an external impact on the airplane,” American and Russian officials attempted to dampen speculation about a possible terrorist attack, but didn’t deny the possibility, either.

All 224 people aboard the Metrojet Airbus A321-200 were killed when it dropped out of the sky into the Egyptian desert this weekend. An ISIS-affiliated militant group in Sinai claimed responsibility.

James Clapper, the U.S. director of national intelligence, said that there was not yet any direct evidence of terrorist involvement—but that it was still possible. “It’s unlikely,” he told reporters. “But I wouldn’t rule it out.”

“No versions could be excluded,” Dmitry Peskov, a spokesman for Russian President Vladimir Putin, said.

Alexander Neradko, Russia’s top aviation official, deemed Metrojet’s statement premature and unfounded. “Such a statement is premature and is not based on any real facts,” Neradko said, according to the New York Times. “Much more work will have to be done on a detailed study of the plane’s constructive elements; flight recorders will have to be deciphered and analyzed.”

But also: “The Egyptian commission is conducting the investigation, and is giving no records and transcripts, be it of the flight recorders or on-ground recorders or radar data, to anyone.”

Robert Galan, a French aviation expert, told the Associated Press that two possibilities followed from Metrojet’s claim. “Either a bomb was placed during the stopover and programmed to explode after takeoff, or a mechanic sabotaged the plane,” he said. “These are the two most probable hypotheses.”

The Guardian reports that the ISIS group’s original statement did not claim to have shot the plane down. According to regional security expert Zack Gold, “A legitimate Isis-supporting [social media] account in Sinai said: ‘Why is everyone talking about shooting it down, why is no one talking about a bomb or suicide bomber on board’?”

“The group does not have a history of major fabrications, but at the same time it’s curious that they would make this claim without providing any kind of evidence,” Gold said.

“They have military capabilities, but to carry out this kind of terrorism [on a plane] they would have to display organisation they haven’t shown [before].”

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