ST. PETERSBURG, Russia » Only an external impact could have caused a Russian plane to dive into the Egyptian desert, killing all 224 people on board, its Russian operator said today, adding to a series of confusing statements from investigators that left unclear why the plane broke up in mid-flight.

In Washington, James Clapper, the U.S. director of national intelligence, said that while there is no direct evidence of any terrorist involvement yet, it couldn’t be excluded that the plane was brought down by Islamic State extremists in Sinai.

"It’s unlikely, but I wouldn’t rule it out," he told reporters in Washington.

Metrojet, the Russian carrier that operated the crashed jet, firmly denied that the crash could have been caused by either equipment failure or pilot error.

"The only possible explanation could be an external impact on the airplane," Metrojet’s deputy director Alexander Smirnov told a news conference in Moscow.

When pressed for more details about the type of impact and what could have caused it, Smirnov insisted that he was not at liberty to discuss details because the investigation was ongoing. He also did not explain whether he meant something had hit the plane or that some external factor had caused the crash.

Asked if the plane could have been brought down by a terror attack, he said only that "anything was possible."

Russia’s top aviation official, Alexander Neradko, chided the company for jumping the gun on the investigation.

Neradko, speaking in televised comments from Egypt, decried Metrojet’s comments as "premature and not based on any real facts." He said it would only be possible to draw firm conclusions about the crash after experts have studied the scattered fragments of the plane in Sinai and the content of its black boxes.

The Airbus A321-200 was flying at 31,000 feet over the Sinai Peninsula when it crashed just 23 minutes after taking off from the Egyptian Red Sea resort of Sharm el-Sheikh en route to St. Petersburg.

Smirnov described the A321-200 as a reliable aircraft that would not fall into a spin even if the pilots made a grave error because its automatic systems would correct crew mistakes.

Viktor Yung, another deputy director general of Metrojet, said the crew did not send a distress call and they did not contact traffic controllers before the crash.

Egyptian officials have offered conflicting accounts on whether or not the plane issued any distress calls.

Neradko said the large area over which the plane’s debris were scattered indicated that it had broken up at high altitude, but he refrained from comment on any possible reason for the crash pending the probe.

The flight recorders will provide key information about the plane’s flight parameters and the operation of its key systems.

Experts say that planes could break up in midair usually because of one of three factors: a catastrophic weather event, a midair collision or an external threat, such as a bomb or a missile.

A local affiliate of the extremist Islamic State group has claimed it brought down the aircraft, which crashed in the northern Sinai where the Egyptian military and security forces have battled militants for years. Russian officials have dismissed that claim as not credible.

Clapper, the U.S. director of national intelligence, noted that the Islamic State group has a significant presence in Egypt’s Sinai Peninsula.

British military analyst Paul Beaver said he thought the crash was most likely caused by a bomb on board as the IS hasn’t been known to possess missile systems capable of striking passenger planes at cruising altitude.

"I’m pretty convinced that ISIS doesn’t have a ‘double-digit’ SAM (surface-to-air missile) that is necessary to go up as far as 31,000 feet," he told The Associated Press, using an alternative acronym for the terror group. "That’s a very serious piece of equipment, and I don’t think they have that sophistication."

He also said the Sinai desert is well-scrutinized by intelligence agencies, so a missile system would have been seen.

Robert Galan, a French aviation expert, says that Metrojet’s claim of an "external impact" pointed to two possibilities: a bomb or sabotage. Sabotage would require familiarity with the electrical or fuel systems of the A321-200, but hiding a bomb would need less knowledge, he added.

Galan said analysis of the plane’s black boxes will not confirm either a bomb or sabotage, as it records only the pilots’ communications and technical readings. But he said investigators could know within 48 hours whether a bomb downed the jet, because the debris would show traces of explosives.

The Irish Aviation Authority says the plane that crashed was registered in Ireland, and regulators there found its safety documentation in order earlier this year.

A Russian government plane early today brought 130 bodies and 40 body parts of the victims to St. Petersburg, the destination of the crashed flight. The city, awash in grief for its missing residents, is holding three days of mourning through Tuesday.

Family members of crash victims have already given DNA samples to speed up the identification process.

At the crash site in Sinai, emergency workers and aviation experts from Russia and Egypt swept across the barren terrain today, searching for more victims and examining the debris for more clues as to the cause of the crash.

Teams finished combing a 7.7-square mile area for bodies by the afternoon and expanded their search to a 11.6 square mile area. Russian Emergency Situations Minister Vladimir Puchkov promised they will not rest until all the remains of the victims are found.

In his first public appearance since Saturday’s crash, Russian President Vladimir Putin described the crash in the Sinai as an "enormous tragedy" and said his thoughts are with the families of the victims.

Mourners have been coming to St. Petersburg’s airport since Saturday with flowers, pictures of the victims, stuffed animals and paper planes. Others went to churches and lit candles in memory of the dead.

Russia held a nationwide day of mourning Sunday and flags flew at half-staff across the country.

Vasilyeva reported from Moscow. Katherine Jacobsen in Moscow, and Ken Dilanian in Washington contributed to this report.