By Stephen Dalziel BBC Russian affairs analyst

One jet was bound for Sochi, where Putin has met Chechen allies

There are enough pointers towards foul play to make this the number one suspicion in the minds of investigators and the public alike.

Using planes for terrorism immediately smacks of the attacks on the US on 11 September 2001.

Witnesses report one plane exploding in mid-air. The other sent out a distress signal - some reports say a hijack signal - seconds before it disappeared from the radar.

Key Chechen ballot

Furthermore, the second aircraft was heading for Sochi, where the Russian President, Vladimir Putin, was on holiday.

And on Sunday there will be elections for a new president for the Chechen Republic - elections which have been dismissed as a sham by many Chechens.

Chechens elect a new president this weekend

For nearly 10 years, Russia has been engaged in a brutal military conflict in Chechnya. There was a lull in actual fighting after the Russian army was chased out, its tail between its legs, in June 1996.

Since September 1999 Mr Putin has made it clear that he wants to bring an end to the conflict - but one which leaves no one in any doubt that Chechnya is part of the Russian Federation.

As the Chechen conflict has ground on over the last five years, there has been an escalation in bombings and acts of terror.

In October 2002, a theatre was seized in Moscow, and 800 people taken hostage; 130 died when troops stormed the building.

There have been suicide bombings in the Russian capital.

A bomb exploded on a crowded metro train during the rush hour one morning in February this year. And the Chechen President, Akhmad Kadyrov, was blown up in Grozny while he reviewed a parade to mark the USSR's Victory Day on 9 May.

Possible revenge

Mr Putin has a reputation for hitting back hard in war-torn Chechnya each time Chechen rebels are blamed for an act of terror.

Some commentators have suggested that using aircraft is a tactic of al-Qaeda, rather than the Chechen rebels.

In the Chechen conflict of 1994-1996, the Chechens did not use suicide bombings. But just as they adopted that tactic later, so they may have learnt the "usefulness" of aircraft for acts of terror.

The Russian authorities may conclude - publicly, at least - that the two aircraft were lost due to pilot or technical error.

But the chances of such an event happening - involving two recently serviced aircraft that had just taken off from the same airport - seem so slim as to be negligible.

Chechnya has largely slipped off the world's news agenda. If the downing of the two Russian planes is directly linked to the Chechen conflict, that conflict will be back in the public gaze, briefly at least - just as it was when Mr Kadyrov was blown up in May.