Terrorism, human error or mechanical mishap could explain why two Russian planes crashed almost simultaneously, killing all 89 people on board, Russia's top investigator said yesterday.

Prosecutor General Vladimir Ustinov told President Vladimir Putin he had no clear view of what happened to the planes, which took off from Moscow's Domodedovo airport late on Tuesday around an hour apart for two different destinations.

They crashed within minutes of each other. "We are examining a number of versions, among them a terrorist act, and human and technical factors," Mr Ustinov told Mr Putin during a meeting with the heads of the FSB security service and the Emergencies Ministry shown on television.

A Tu-134 flying to Volgograd went down near the town of Tula south of Moscow. Within minutes and 800 kilometres away, a Tu-154 bound for the Black Sea resort of Sochi crashed near the southern town of Rostov-on-Don.

The owner of the Tu-154, Sibir Airlines, said the pilots had triggered a hijack alert just before their plane crashed. It was carrying 46 passengers and crew.

"The message was generated right before all contact was lost with the plane and it disappeared from radar screens," Russia's number two airline said in a statement. The wide distribution of wreckage suggested the plane exploded in mid-air, it said.

Volga-Aviaexpress, a small regional carrier which owned the Tu-134, said the crew did not report any problems on board before the plane crashed with 43 passengers and crew. The Emergencies Ministry later said 44 people were aboard.

Aided with cranes, investigators sieved through high grasses in almost identical countryside near the two sites. Huge slabs of twisted metal, seats and the odd piece of clothing were scattered over dozens of kilometres.

Mr Putin, who broke off his summer holiday in the Black Sea resort of Sochi to return to Moscow, had ordered the FSB to investigate the case, which it is normally only asked to do if terrorism is suspected.

Fear of attacks in Russia is already high ahead of Sunday's presidential election in restive Chechnya. Separatist rebels have vowed to disrupt the poll.

Analysts said the chance of two planes crashing within minutes must have been more than a coincidental malfunction.

"It's freaky if it is not sabotage," said David Learmount, operations/safety editor at Flight International magazine.

The crashes came against a backdrop of violence in Chechnya, where Moscow has been battling separatists for a decade. Rebels launched a major raid in the local capital Grozny last week.

Moderate Chechen separatists denied any role in the crashes. Asked if his group was responsible for the crashes, Akhmed Zakayev, a spokes-man for Chechnya's separatist leader Aslan Maskhadov, told Reuters in London: "Of course not."

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