Explosions 'led to Russia crashes'
Russia holds a day of mourning for the 89 people who died in the crashes.
Russian search and rescue officials find the crash sites.
MOSCOW, Russia (CNN) -- Russian investigators said Monday that explosions led to the near-simultaneous crashes of two Russian jetliners last week.
The Federal Security Service (FSB) is focusing its investigation on whether acts of terrorism brought down the jets after traces of the explosive hexogen were found in the wreckage of both planes.
Eighty-nine people died in the crashes Tuesday.
Hexogen, when mixed with nitroglycerin, forms a plastic explosive similar to C4 and has been used by Chechen rebels in attacks on Russian soil in the past.
Although Russian authorities are taking great care not to assign blame in the crashes, at the center of the investigation are two Chechen women who were aboard the planes.
Authorities confirm that no friends or relatives have come forward to claim the remains of either woman, and they are the only two passengers on either flight that have not been inquired after.
The FSB also said that the women purchased the tickets for their flights at the last minute.
The crashes took place ahead of Sunday's regional election in the rebellious southern territory of Chechnya, where Russian troops have battled separatist guerrillas since 1994.
Chechen voters elected a new president, Alu Alkhanov, following the assassination in May of then-President Akhmad Kadyrov. (Full story)
Through a spokesman, Chechen rebel leader Aslan Maskhadov has denied any involvement in Tuesday's plane crashes.
The crashes raised concerns that security at Russian airports is too lax, leading Russian President Vladimir Putin on Friday to propose bringing all airport security under the aegis of the Russian Interior Ministry.
Currently, airport security at Moscow's Domodedovo Airport, where the airplanes departed, is the responsibility of private contractors.
Most other Russian airports also use private contractors for security.
Authorities also said Saturday that the flight data recorders from the planes may not be of much use in the investigation. Apparently the devices on both planes either malfunctioned or shut down before the crashes.
Perhaps the most notorious attacks involving Chechen rebels in Russia occurred in September of 1999 when apartment buildings in Moscow, Buynaksk and Volgodonsk were bombed resulting in the deaths of nearly 300 people. All of the bombs used were found to contain hexogen.
Other attacks credited to Chechen rebels have killed hundreds. They include:In June of 1995, Chechen rebels seized a hospital in southern Russia, about 100 people died.In July of 2000, guerrillas launch five suicide bomb attacks on Russian security bases, the deadliest attack killed 54 people.In October of 2002, 129 hostages and 41 guerrillas were dead after Russian troops stormed a Moscow theater where rebels had taken 700 captives. Many of the hostages were killed by the gas Russian troops used to knock out Chechens.In December of 2002, about 80 people were killed when Chechen suicide bombers rammed vehicles into the local government headquarters in Grozny, bringing down a four-story building.In May of 2003, 59 people were killed as two suicide bombers drove a truck full of explosives into a government complex in northern Chechnya.In July of 2003, two female suicide bombers killed 15 people when they blew themselves up at an open-air rock festival in Moscow.In February of 2004, 40 were killed and 100 injured when a suicide bomber detonated a bomb on a Moscow metro train.