Traces of explosive have been found amid the wreckage of one of two Tupolev airliners that crashed on Tuesday, Russian officials say.
Both crash sites are being searched for clues
The FSB security service said at least one of the almost simultaneous crashes was a "terrorist act".
Details of the discovery came after an Islamic group claimed responsibility for the crashes in a website statement.
Investigators are still working to decode the flight data recorders from the crashes, which left 89 people dead.
Russian aviation officials have confirmed that one of the planes, a Sibir Airlines Tu-154, sent out a hijack alert just before it crashed, Russian media report.
The Tu-154 and a Tu-134 jet crashed within minutes of each other over southern Russia, coming down about 800km (500 miles) apart.
The FSB says the traces of explosive were found amid the debris of the Tu-154, which was flying to the Black Sea resort of Sochi when it disappeared from radar shortly after the pilot pressed the SOS button.
The same explosive, hexogen, was apparently used in a series of apartment bombings in 1999 that killed around 200 people.
In a website statement on Friday, a group called the Islambouli Brigades said it had five people on board each aircraft. It warned this act would be followed by others "until the killings of our Muslim brothers in Chechnya cease".
The crashes came just days before a presidential election in war-ravaged Chechnya, where separatists recently stepped up attacks on Russian forces and their local allies.
The FSB said it had identified "a number of people with possible links to the terrorist act". Officials are investigating the identity of two female passengers - one on the Sochi flight who has been confirmed as a resident of Chechnya.
But no relatives have yet come forward to claim her body.
Transport Minister Igor Levitin, who heads the government commission investigating the crashes, said on Thursday that more time was needed to decode the "black box" flight data recorders.
"Not all the flight recorders are in a condition that would allow them to be read immediately," he said before flying to Tula Region, where one of the planes crashed.
"Today and tomorrow we will work on them in order to bring the tape to a condition that will allow us to read what happened."
President Vladimir Putin's envoy to southern Russia told reporters that the black boxes had "practically switched off immediately".
Vladimir Yakovlev said this was "probably more confirmation... that something had happened very quickly".
Investigators have continued to sift through the wreckage scattered over fields in Tula and Rostov Region.
Russian flags flew at half-mast on Thursday and light entertainment was withdrawn from theatre and television schedules as part of a day of national mourning.
Passenger lists indicate that all the victims were Russian, apart from one Israeli.
The black boxes are said to be in poor condition
Russia's newspapers have largely poured scorn on the official line that the cause of the crashes was probably technical or human error.
"Despite the titanic efforts of state television, we are not all imbeciles just yet," said an editorial in Moskovsky Komsomolets.
"Now Russia has its own 11 September," said the headline in Nezavisimaya Gazeta.
The victims' families are to receive 112,000 roubles ($3,800) each in compensation - unless it is proven that terrorism was to blame, in which case they would receive less.