Up until now, Russian officials said they were looking at a number of possible causes for the air crashes.
But to, a spokesman for the Federal Security Service (FSB), announced what many had already suspected.
Sergei Ignatchenko told ITAR-TASSs that, according to a preliminary investigation, "at least one of the plane crashes" was the result of a terrorist attack.
Around the same time, another FSB spokesman, Nikolai Zakharov, announced the discovery of traces of explosives in the wreckage of that plane.
"After searching through the wreckage of the Tu-154 plane, investigators found traces of explosive material," Zakharov said. "Preliminary analysis showed the substance to be hexogen, but additional analysis is being carried out. The wreckage of the Tu-134 plane is being analyzed, but there are no new results."
The two planes crashed within minutes of each other after taking off from the same Moscow airport, Domodedovo.
A Tu-134 was bound for Volgograd when it crashed near the town of Tula, south of Moscow.
A Tu-154, bound for Sochi on the Black Sea, crashed near the southern city of Rostov-on-Don.
Reports say that plane managed to send a distress signal before crashing. The distress signal -- plus the extraordinary coincidence of two planes crashing within minutes of each other -- raised suspicions of possible terrorist attacks.
Heightening those suspicions is the fact that the crashes came just days before a presidential election in Chechnya, where Russian forces have been fighting separatist rebels.
An Islamist group calling itself the Islambuli Brigades said its followers had hijacked and brought down the planes to avenge the killing of Muslims in Chechnya.
Earlier today, there was the first claim of responsibility. An Islamist group calling itself the Islambuli Brigades said its followers had hijacked and brought down the planes to avenge the killing of Muslims in Chechnya.
The FSB is declining to comment on that claim. But Zakharov said investigators have leads on a number of suspects.
"As a result of the investigations, the FSB identified a number people who could have been involved in a terrorist act," Zakharov said.
Reports say investigators are looking into the backgrounds of two female passengers with Chechen-sounding surnames. Each plane carried one of the women and neither has been enquired about by relatives.
Akhmed Zakaev, a spokesman for Chechen separatist leader Aslan Maskhadov, said on 25 August that Maskhadov was not connected in any way to the crashes.