At least eight people were killed and more than a dozen injured on Monday in twin car blasts outside a court building in the southern Russian city of Makhachkala in the restless region of Dagestan.
The local investigative committee said that "within the space of a few minutes, two cars blew up. At least eight people died. The number of people injured is being checked."
Officials added that at least two dozen cars were damaged by shrapnel.
Dagestan's police department said the second bomb went off after law enforcement officials had arrived on the scene to investigate the first explosion.
The blasts happened within 15 minutes of each other outside a building used by court bailiffs.
ITAR-TASS quoted one police source as saying that this may have been an assassination attempt against a senior bailiff's office representative.
Reports said that two law enforcement officials and a bailiff's office employee had died.
"At the moment we know... of 15 to 16 injuries," ITAR-TASS quoted the unnamed police source as saying.
Another investigator said the bombs may have been remotely set off.
The Caspian Sea republic of Dagestan is one of Russia's most violent regions. It is also home to the parents of the Boston bombing suspects Dzhokhar and Tamerlan Tsarnaev.
The predominantly Muslim republic suffers from poverty and well-organised crime syndicates as well as an influx of armed guerrillas from the neighbouring republic of Chechnya.
War-wrecked Chechnya fought two brutal post-Soviet campaigns against federal troops and has now been placed under Moscow's control with the help of iron-fisted ruler Ramzan Kadyrov.
But Moscow has struggled to exert its influence on Dagestan and analysts believe that its local leadership remains weak.
Dagestan is frequently targeted by smaller explosions and acts of violence against police officers.
Bigger attacks are rarer and have been the focus of a massive security crackdown ordered by President Vladimir Putin.
The security measures have recently helped save Moscow from the big bombings that shattered metro stations in 2010 and even a major international airport in 2011.