MOSCOW - Officials said Saturday they had found traces of an explosive on the wreckage of the second of two Russian airliners that crashed just minutes apart earlier this week.

That would indicate terrorists caused both aviation tragedies.

Evidence of the explosive hexogen were found on the Tu-134 jetliner that crashed Tuesday in the Tula region, about 100 miles south of Moscow, said Sergei Ignatchenko, spokesman for the Federal Security Service.

Discovery of the explosive residue on the second jetliner was revealed one day after authorities said residue of the same explosive material was found on the wreckage of a Tu-154 that crashed farther south in Russia, near the town of Rostov.

Officials said the explosive residue showed terrorists brought down that plane.

Both planes crashed Tuesday night after taking off from Moscow's Domodedovo airport, one of Russia's most modern and sophisticated air hubs. The findings of explosives indicated significant weaknesses in security for the air transport network that spans the vast country.

The crashes took place just five days before residents of the warring republic of Chechnya were to go to the polls to choose a president in an election that the Kremlin portrays as a step toward restoring civil order in the region.

Officials had warned Chechen separatist rebels could resort to terrorism to try to undermine the Sunday voting. The Kremlin refuses to negotiate with the rebels.

A Web site connected to Islamic militants claimed the crashes were retaliation for Russia's ongoing war in Chechnya, and Russian officials said they were investigating the backgrounds of two female passengers with Chechen surnames - one on each of the planes.

Several suicide bombings in recent years have been blamed on Chechen women who lost husbands or brothers in the war and chaos that have plagued the southern republic for most of the past decade.

Both women had booked tickets on the flights at the last minute and were the only victims whose relatives have not contacted authorities, officials said.