Moscow ? Authorities said they were focusing on two female Chechen passengers who were aboard the Russian airliners that crashed Tuesday night, while investigators said they had found traces of explosives on the wreckage of one the flights, raising suspicions that the two planes were brought down by suicide bombers.

Sergei Ignatchenko, chief spokesman for the Federal Security Service, or FSB, told reporters that at least one of the aircraft had been the target of a “terrorist attack,” citing the discovery of small amounts of hexogen, a high explosive, on wreckage at one of the crash sites.

Other investigators confirmed they were seeking more information about the two Chechen women, each a passenger on one of the Soviet-era passenger jets. Authorities said both women booked tickets on the flights at the last minute, and neither woman’s relatives had contacted authorities.

Chechen women strapped with explosives have carried out a dozen or more attacks on Russian targets over the past four years, killing more than 200. The bombers have been dubbed “black widows” because some were the wives or relatives of separatist fighters in the war-ravaged republic killed by Russia’s federal forces.

FSB officials told the Interfax news service Friday that they were searching for relatives of Amanty Nigayeva, a 27-year-old Chechen. She purchased a ticket on the Volga Avia Express flight an hour before the plane departed from Moscow’s Domodedovo Airport, heading south to the city of Volgograd.

The twin-engine, Tupolev 134 jet airliner disintegrated in mid-flight about 120 miles southeast of Moscow. Witnesses reported three large explosions preceded the crash.

Investigators said they weren’t certain Friday why Nigayeva was headed to Volgograd, and that no one was waiting for her at the airport Tuesday night.

Investigators also are seeking friends and family of another Chechen woman, identified only as S. Dzhebirkhanova.

Dzhebirkhanova registered for Siberian Airlines’ Tupolev 154 flight under a pseudonym, authorities said, and she also purchased her ticket shortly before her flight took off.