Photographs of a young woman, obtained by Reuters from a law-enforcement official in Dagestan, showed her dressed in a black hijab and holding a grenade.
Another photograph showed the woman holding a pistol. The same photograph was published in the Kommersant newspaper on Friday.
The source, who asked not to be identified because of the sensitivity of the issue, named her as Dagestani-born Dzhennet Abdurakhmanova, the widow of 30-year-old Umalat Magomedov, a prominent insurgent killed by Russian forces on December 31.
Abdurakhmanova also used the name of Dzhanet Abdullayeva, the source said.
Magomedov, who was shown in the photographs holding a pistol, styled himself as the "Emir of the mujahideen of the Vilayat Dagestan," a local Islamist group, the source said.
The Russian Prosecutor-General's main investigations unit later identified the same woman as the bomber.
"A native of Dagestan, Dzhanet Abdullayeva, born in 1992, detonated explosives at the Park Kultury metro station," it said in a statement, giving no further details.
Officials said two female suicide bombers -- known in the Russian media as "Black Widows" -- killed at least 40 people on packed Moscow metro trains during the rush hour on Monday.
The first bomb tore through a metro train just before 8 a.m. as it stood at the Lubyanka station, close to the headquarters of the FSB. A second bomb was detonated less than 40 minutes later in a train waiting at the Park Kultury metro station.
The suicide bombings in Moscow and Dagestan follow a surge of violence over the past year in the patchwork of North Caucasus republics, where Russia has fought two wars against Chechen separatists since the 1991 fall of the Soviet Union.