Several versions of the blast that occurred in the Moscow metro on Friday morning, including a suicide bombing, have become available.
Experts have also prepared a composite image of a man who might have been involved in the blast.
"It is too early to speak about what was blown up, when it was done and by whom before the scene of the blast and the remains of the explosive device have been examined," a member of the group of experts working at the scene told Interfax.
"We are currently considering several versions, including the possibility that the explosive device was set off by a suicide bomber or that an explosive substance went off unplanned. There are indications that the explosive device was in a suitcase or bag," the expert said.
A Moscow metro video-camera recorded a woman suspected of being a suicide bomber and her alleged accomplice standing on a metro station platform carrying two suitcases, sources at the headquarters investigating the metro explosion told Interfax. Police are searching for a clean-shaven man aged 30-35 who appears to be a native of the Caucasus.
Experts are currently studying the scene and are gathering pieces of the explosive device, the expert said. Forensic experts are working on identifying people killed by the blast. The images recorded on the security cameras in the metro will also be analyzed.
One of the most probable versions is that the terrorist act was perpetrated by a suicide bomber, a source in the Main Interior Affairs Department of Moscow said.
"At this time, we have no information as to who it could be, a man or a woman," the source said.
Sources in the headquarters set up to investigate the disaster said, citing witnesses, that the explosion might have been perpetrated by a female suicide bomber.
Law-enforcement authorities estimated the blast killed 39 and injured 122 people.