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 (search).

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. A total of 90 people were killed in the disasters.

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 predominantly Muslim 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 that 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.

On Saturday, the newspaper Izvestia cited a Chechen village leader, Dogman Akhmadov, as saying that the brother of one of the suspect women had disappeared three or four years ago and was believed to have fallen victim to Russian forces who are widely accused of civilian abductions and summary executions in Chechnya.

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. One of the women gave only her surname and first initial in booking the ticket, according to reports.

The Transport Ministry said Saturday that passengers on domestic flights now will be obliged to show full passport details on their tickets, ITAR-Tass reported, citing an unidentified ministry official who said the measure will "make the process of documenting passengers and baggage more transparent and controlled."

Russian citizens have separate passports for internal and foreign travel.

The first official confirmation that terrorists infiltrated Russia's civil aviation system — a vital industry in this vast nation — otherwise prompted only a muted official response, with Russian authorities avoiding drastic measures such as closing airspace or grounding flights.

President Vladimir Putin (search) made no public comment on the discovery of traces of explosives three days after what one major newspaper called Russia's Sept. 11.

Hexogen was identified as the explosive in a series of 1999 apartment-building bombings that killed some 300 people in Moscow and other cities and that were blamed on Chechen separatists.

The bombings led in part to Putin's decision to send troops back into the region. Despite his high popularity ratings, Putin's presidency has been marred by persistent fighting in Chechnya and deadly bombings beyond its borders.

A Web site statement that appeared Friday was signed the "Islambouli Brigades" and claimed responsibility for the crashes, warning that they marked just the first in a series of planned operations. The claim's veracity could not be confirmed.

"Russia's slaughtering of Muslims is continuing and will only stop when a bloody war is launched," the statement said. It said five "mujahedeen" — holy fighters — were aboard each plane.

Russian officials have repeatedly contended that the rebels who have been fighting Russian forces in Chechnya for nearly five years receive help from foreign terrorist organizations, including Al Qaeda.

Friday's claim did not refer to Al Qaeda, but a group called "the Islambouli Brigades of Al Qaeda" claimed responsibility for last month's attempt to assassinate Pakistan's prime minister-designate.

Paul Duffy, a Moscow-based aviation expert, told Associated Press Television he found it "hard to believe" that five attackers were aboard each plane, but said "there is no doubt that they had one at least on each aircraft."

Representatives of Chechen rebel leader Aslan Maskhadov (search) have denied connection to the crashes. But Maskhadov, who led Chechnya during its 1996-99 period of de-facto independence, is believed to control only a small portion of Chechnya's fighters.