Experts have found explosives in both Russian jets which crashed simultaneously this week, investigators said yesterday, supporting theories that bombs downed the aircraft before elections in volatile Chechnya.

The FSB security service said on Friday search teams had turned up traces of explosives in the first of the planes which crashed on Tuesday, killing 90. Its new disclosure was on the eve of a poll certain to return a pro-Moscow Chechen president.

"Additional examination of the fragments of the Tu-134 aircraft which crashed on Tuesday... has revealed traces of hexogen," an FSB spokesman said by telephone. Hexogen, more widely known as RDX, was used in previous attacks blamed on Chechen militants.

Investigators had been pursuing leads linked to terrorism in the crashes ahead of Sunday's election, dismissed as a farce by Islamist separatists fighting Russian rule for a decade.

FSB spokesman Sergei Ignatchenko said Russia was "studying international experience in fighting terrorism in air transport". Russia was examining the Israeli system, "which is considered the best in the world," he told Russian television.

Officials have carefully avoided any suggestion that Chechen militants were behind the crashes. But Russian media have speculated that two passengers, believed to be Chechen women, blew up the planes in the run-up to today's election. At least one passenger aboard the two flights remained unidentified.

Security was high in Chechnya for the election. Some 14,000 Chechen police patrolled the streets, alongside Russian forces. Outside one polling station police were charged with 24-hour security shifts, stopping cars from parking within 15 metres of the building. Kremlin-backed candidate Alu Alkhanov is virtually certain to win the poll but many doubt his ability to rein in rebels.

As cranes removed fragments of wreckage from the crash sites, Russia's transport minister also toughened air security measures and vowed to prevent any recurrence.

Safety measures, previously undertaken solely by airports, would now be shared with the Interior Ministry, he said. Passengers would have to submit passports when buying tickets and aircraft would be equipped with modern alarm systems.

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