Aviation experts say the circumstances of Russia's double air disaster look suspicious.
Security measures in Russian airports have come under scrutiny
The odds on two airliners crashing almost simultaneously by accident after leaving the same airport are extremely slim, they say.
The two Russian jets, carrying 89 passengers and crew, crashed within minutes of each other, after leaving Moscow's Domodedovo airport on Tuesday night.
Russian security officials say no sign of terrorism has been found at either crash site.
Such accidents could happen, airline analyst Mike Boyd says, but some of the circumstances point to the possibility of a deliberate attack.
"In this day and age, when we're looking at terrorist threats, an election in Chechnya and President [Vladimir] Putin vacationing in the Black Sea - you put all that together and it does not look good," he told the BBC World Service's The World Today programme.
Apart from the 11 September 2001 hijackings, "I don't know of any example of two airplanes taking off from the same airport, going in the same general direction - south - and disappearing at almost the same time," he added.
According to aviation safety expert Chris Yates of Jane's Transport, extra attention to airport security in North America and Western Europe since 9/11 has not been matched widely elsewhere.
"I don't see any evidence of a uniform level of security worldwide. In certain parts of the world the application of security and provision of security is pretty patchy," he told BBC Radio 4's Today programme.
About 60 Tu-154s have crashed since they were introduced in 1968
Forensic experts working on the terrorism theory would be looking for evidence that the jet's fuselage had been blown apart by an explosion, he said.
Russian airline safety has been steadily improving, however, despite a decline immediately after the collapse of the Soviet Union in 1991, according to aviation expert David Learmount of Flight International magazine.
One of the planes in question, the Tupolev 154, was the most commonly used plane in the former Soviet Union, he told BBC News 24.
"The aeroplane itself is built like a tank, it's a very, very good airplane. But because there are so many around, if an accident happens in Russia than there's a chance that it's going to be one of those," Mr Learmount said.
But to have two seemingly unconnected air accidents in the same country on the same day was "very, very strange indeed," he added.