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A last-second noise heard on a cockpit recording has become the focus of an Egypt-led probe into the crash of a Russian passenger jet in the Sinai peninsula last week as investigators said they don’t yet have enough evidence to conclude that the incident was caused by a blast.

The international group of investigators will use spectral analysis to identify the noise, Ayman El-Mokadem, who leads the probe, said at a news conference in Cairo on Saturday. The committee is considering “all possible scenarios,” while parts of the Airbus are still missing and the cause of its mid-air breakup hasn’t yet been determined, he said. El-Mokadem didn’t specify a time frame for the investigation to conclude.

Debris from the crashed Russian plane lies on the sand at the site of the crash in Sinai. 2015.

The two nations with most at stake in the investigation have been reluctant to brand the incident a terror attack. If confirmed, the bomb theory that has gained traction especially among U.S and U.K. officials would highlight the risk of reprisals Russia is facing for its bombing campaign in Syria. The crash that killed all 224 passengers aboard a flight to St. Petersburg from the Sinai resort town of Sharm el-Sheikh also threatens to torpedo Egypt’s tourism industry, a pillar of the nation’s economy.

“The committee noted media reports and analysis, some of which claimed to be based on official intelligence which favors a certain scenario for the cause of the accident,” El-Mokadem said. “The committee was not provided with any information or evidence in this regard. The committee urges the sources of such reports to provide it with all information that could help us to undertake our mission.”

French officials familiar with the investigation said a “violent, sudden” explosion caused the crash of the Metrojet Airbus A321, the BBC reported. U.K. and U.S. officials have also said that evidence from intelligence sources suggests that a bomb placed aboard the plane brought it down.

President Vladimir Putin on Friday halted all Russian flights to Egypt. The Russian Federal Air Transport Agency Saturday said it would send 44 aircraft to two Egyptian Red Sea resorts to evacuate Russian nationals, AFP reported.

A Russian member of the commission probing the incident said it was still too early to determine the nature of certain noises recorded before the crash, the Moscow-based news service Tass reported before the briefing in Cairo.

Bomb Theory

The technical portion of the investigation is unwinding, as aviation experts reach a strong presumption that the crash was caused by a bomb, two people familiar with the matter said.

Investigators have found no element that would signal a mechanical incident, the people said, asking not to be named discussing confidential information. While there’s no certainty at this point of what caused the crash, a sudden rupture of the plane for technical reasons is extremely unlikely, the people said. The probe will shift into a criminal investigation to look into a potential attack, the people said.

Spectral analysis of the noise heard on the recording just before it cut off will be carried out by specialized labs, El-Mokadem said. While such examination can provide clues such as sound-wave spikes and sonic signatures, they are typically insufficient to alone determine the cause of the crash.

“The sound alone is not the sole evidence, there should be a large number of evidence to determine a certain conclusion,” El-Mokadem said.

Travel Advisory

As the investigation continues, unease is growing over travel to the peninsula, a popular tourist destination.

The U.K. Ministry of Transport said about 1,500 people flew home to Britain on Saturday, about the same number that returned Friday from Sharm el-Sheikh. The government said additional security at the resort has caused flight delays, “but our top priority will continue to be the safety of British nationals.”

“We are working closely with the Egyptian authorities, U.K. airlines and tour operators to have passengers on flights and returned to the U.K. as soon as possible,” the government said in a statement Saturday.

After the Russian flight stop and an earlier U.K. warning against all but essential trips to the region, several Nordic nations also advised their citizens against traveling and staying in Sinai.

“Until the situation is further cleared up, non-essential travel to Sharm el-Sheikh is also discouraged,” the Norwegian foreign ministry said in a statement on Saturday. Denmark and Finland issued similar warnings. The three nations halted flights to the city, the AP reported.

— With assistance by Mahmoud Habboush, Andrea Rothman, Alaa Shahine, Marie Mawad, and Todd White

(Updates with Russian evacuation in sixth paragraph, UK statement in 14th.)