Here’s what we know about the Russian plane crash that killed all 224 people onoard Oct. 31. (Monica Akhtar/The Washington Post)

Scraps of suggestive but inconclusive evidence surfaced Tuesday in the fourth day of a tense investigation into a Russian plane’s falling over Egypt’s troubled Sinai Peninsula on Saturday, killing all 224 people aboard and scattering debris over seven square miles of desert.

Investigators have not said whether the plane disintegrated in midair because of a technical malfunction, an act of terrorism or some other reason. They have begun reviewing information from the plane’s flight recorders, but a report could take two to four weeks to produce, an Egyptian government spokesman said.

A U.S. satellite registered a “heat flash” about the time that the plane crashed, a U.S. official said Tuesday, speaking on the condition of anonymity because he was not authorized to discuss the information publicly. Although that could mean a bomb or a fuel tank aboard the plane exploded, he said it was too soon to jump to conclusions and that the heat flash may have been triggered by the breakup of the plane itself.

The governments of Egypt and Russia, the two countries closest to the investigation, have sought to quash speculation about what brought down the plane. In Russia, news agencies that serve as clearinghouses for government statements have released new clues about the crash, but those reports cited anonymous sources in the investigation, making them difficult to verify.

Russia’s Interfax news service, citing a source in the investigation, said Tuesday that there were no signs of a malfunction with the plane and that the pilots were chatting normally with air-traffic controllers until four minutes before an “emergency situation occurred on board unexpectedly.”

“In the recordings, sounds uncharacteristic of a standard flight precede the moment of the airliner’s disappearance from radar screens,” the news service reported without elaboration. “The pilots had no time to send out a distress signal.”

Russia’s transport minister, Maxim Sokolov, said Tuesday that Russian experts already had conducted a preliminary inspection of the two flight recorders, commonly known as “black boxes,” and had seen information from Egypt’s flight-control radars. But he did not discuss further details.

Dmitry Peskov, the personal spokesman for Russian President Vladimir Putin, told journalists Tuesday that Russia was looking into the reports of the heat flash, but he gave few other details about the investigation. He chided a reporter for asking whether Russia’s intervention in Syria may have served as a motive for an attack on a Russian plane.

“These hypothetical theories are completely inappropriate,” he said.

But the theories keep coming. A regional news outlet based in the Russian city of Ekaterinburg published a photograph Tuesday of the same plane bearing what appeared to be a scratch or crack in the metal on the aircraft’s tail. The photographer said that he had taken the picture in May and that the defect was not visible on subsequent photographs of the plane.

The Aviation Safety Network reported that the Metrojet plane suffered a “tail strike” in 2001, a type of incident in which an airplane’s tail hits the runway. The damage took three months to repair, but the jet was certified as airworthy this year by regulators in Ireland, where it was registered.

The airline has vigorously defended its safety record, and it provoked a public quarrel with the government Monday when it said that the cause of the plane’s disintegrating in midair could only be a “mechanical impact,” rather than technical issues or pilot error.

The head of Russia’s Federal Air Transport Agency, Alexander Neradko, issued a swift rebuke: “That statement is premature and is based on no real facts,” he said

Determining the reason for the crash is fraught with consequences for Russia and Egypt.

If the aircraft had a technical malfunction, it implicates Russia’s troubled airline industry. If a sophisticated terrorist attack brought down the plane, it implies that aircraft are not safe over Egyptian soil.

The U.S. Embassy in Cairo instructed its staff Tuesday not to travel anywhere in Egypt’s Sinai Peninsula as a “precautionary measure,” pending the outcome of the investigation. The move sparked an angry response on social media from the Egyptian Ministry of Foreign Affairs, a sign of the concern that the perceived threat of terrorism is generating.

“When there is propaganda that it crashed because of ISIS, this is one way to damage the stability and security of Egypt and the image of Egypt,” Egyptian President Abdel Fatah al-Sissi told the BBC in an interview, in a reference to the Islamic State militant group. “Believe me, the situation in Sinai — especially in this limited area — is under our full control.”

Russia began launching airstrikes in Syria about a month ago and is concerned about blowback. The suggestion that the airline was targeted by terrorists, possibly because of the country’s intervention in Syria’s civil war, would mark a high cost for the country’s increasingly assertive foreign policy.

Although an affiliate of the Islamic State in Egypt claimed responsibility for the plane crash, Russian officials were skeptical.

Russia continued Tuesday to bring home the remains of its citizens who died aboard the Airbus 321 Saturday shortly after takeoff from the Egyptian resort city of Sharm el-Sheikh on its way to St. Petersburg.

On Monday, Putin called the accident “a great tragedy.”

Read more:

Militant cells are carrying out more brazen attacks across Egypt

Russian airline official rules out technical error as cause of crash

Today’s coverage from Post correspondents around the world