Experts have found explosives in both Russian jets which crashed simultaneously last week 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.
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 Sunday's election. At least one passenger aboard the two flights remained unidentified.
