Russian government officials responded with a swift rebuke that it was both premature and without foundation to speculate on what caused the crash, wanting to contain speculation - and potential embarassment - over what led to the deadliest civil aviation disaster in Russia's history.

READ MORE: Russian jet broke up in mid air but too early to draw conclusions, official says

On Tuesday morning, Russia's Interfax news service, citing a source in the investigation, said there were no signs of a malfunction with the plane and that 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.”

Metrojet said Monday there were no signs of an“external impact,” however, meaning a missile or other projectile.

Russia continued Tuesday to repatriate the remains of its citizens who died aboard the Airbus 320-200 Saturday shortly after take-off from the Egyptian resort of Sharm el-Sheikh on its way to St. Petersburg, Russia's second largest city.

What happened in those last four minutes of flight remains a mystery.

Several American television stations reported Monday evening that U.S. intelligence satellites had captured a “heat flash” over the Sinai Peninsula at the moment of the crash, signalling some sort of explosion either from a bomb, missile, or from the fuel on board the aircraft.

The answer to the crash is fraught with consequences for both Russia and Egypt.

If it was a technical malfunction of the aircraft, it implicates Russia's troubled airline industry. If it was the result of a sophisticated terrorist attack, it implies that aircraft are not safe over Egyptian soil.

“When there is propaganda that it [the plane] 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 el-Sissi told the British Broadcasting Corporation 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 a little over one 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, is fraught with consequences for Russia.

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 Associated Press reported from Egypt.

An Egyptian government spokesman said Monday it would take two to four weeks to study the black boxes retrieved from the plane and provide a full report on what caused the jetliner to crash.

The head of Russia's Federal Air Transport Agency, Alexander Neradko, said in a television interview from Egypt that investigators have not reviewed the black boxes yet.

“Yes, we know that components of the plane have been thrown over a wide area. That says that the breakup took place in the air, at a high altitude,” he said. “But it is very premature to talk about the reasons. I would like to call on the aviation community to abstain from premature statements,” Neradko said.

While an Egyptian affiliate of the Islamic State seized the opportunity to claim responsibility for the plane crash Saturday, Russian officials remained sceptical.

READ MORE: Investigators begin probe on crashed Russian airliner

Russian President Vladimir Putin called the accident “a great tragedy” Monday.

“Everything must be done to create an objective picture of what happened so that we know what happened and react accordingly,” Putin told Transport Minister Maxim Sokolov during a televised briefing on the investigation.

The claim by the Egyptian affiliate of the Islamic State led some international carriers to re-route flights away from the Sinai, though defence experts have raised strong doubts about whether the Islamic State could have missile systems capable of hitting an airliner at 31,000 feet.

In Washington, James R. Clapper Jr., the director of national intelligence, said that there was no “direct evidence of terrorist involvement yet” but that it cannot be disregarded. “It's unlikely, but I wouldn't rule it out,” Clapper told reporters.

Prevailing wisdom holds that once an aeroplane reaches cruising altitude, it's clear sailing, but there have been at least half a dozen times when a commercial jetliner has fallen to pieces without help from a bomb or a missile.

As the investigation continues, there are unmistakable signs that Metrojet is in the crosshairs.

Investigators this weekend opened a probe for criminal negligence in the crash and searched Metrojet's offices. Russia's Federal Labor Agency announced Monday that the airline had not paid its employees in the past two months, indicating financial problems in the company. And a state-owned television channel broadcast an interview with the pilot's wife, who said her husband had complained about poor maintenance on the plane.

Metrojet's embattled leadership mounted a public defense Monday at a news conference in Moscow.

Alexander Smirnov, the deputy general director of the airline, said no combination of factors, including bad fuel or engine failure, could have led the plane to break up in mid-air. Metrojet officials also said that the plane was regularly reviewed for signs of structural weakening and argued that although the company had withheld wages recently, that did not affect safety standards.

“The only explanation could be a mechanical impact on the aircraft,” Smirnov said. He declined to elaborate as to what such an impact could have been.

Though Smirnov discounted the possibility that the Metrojet plane could have fatally malfunctioned, it would not be unprecedented.

Planes climb to a cruising altitude of six to seven miles above the Earth's surface because there, the air is far thinner, and against that lessened resistance they can fly faster and use less fuel.

When they reach that altitude, however, they must maximise the air pressure in the cockpit and cabin - and that puts stress on any component that has weakened over time.

“Typically, if there was that type of defect, you would expect it to manifest just as it reached the peak” altitude, said Steve Wallace, a former crash investigator for the Federal Aviation Administration.

In 1988, a hole opened in the fuselage of an Aloha Airlines plane in Hawaii, sucking out a flight attendant. Metal fatigue was blamed in 2002 when China Airlines Flight 611 disintegrated after take-off from a Taiwan airport, killing all 225 people on board.

“That aeroplane had an improper repair after a tail strike,” Wallace said, “and I think this Russian aeroplane that was in the accident had had a tail strike.”

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 Metrojet crash comes as Russian airlines are facing an economic crunch that has forced Transaero, the country's second-largest airline, to file for bankruptcy. Russia's aviation authorities grounded Transaero flights recently over concerns that the airline could not maintain safety standards.