Investigators searched for clues Tuesday into why two car bombs were detonated in Makhachkala, killing four and injuring 52 others, as the authorities defused a third car bomb.
Both explosions occurred outside the local headquarters for the court marshals service and appeared to have been detonated by remote control, said the federal Investigative Committee.
No fatalities or injuries were caused by the first explosion Monday. But once police arrived at the scene 15 minutes later, the second bomb detonated, leaving three people dead, according to the local branch of the Investigative Committee.
An employee of the court marshals service later died of his injuries in the hospital, said an Interior Ministry spokesman.
According to preliminary data, most of the injured were law enforcement officials.
"The police were providing a cordon after the first explosion. Because of their actions, the number of civilian victims is minimal," the Interior Ministry spokesman told Interfax.
No one has claimed responsibility for the double blast, and no suspects have yet been identified.
Video footage showing one of the car bombs detonating Monday. (YouTube)
The blasts might have targeted the son of Makhachkala's mayor who heads the local headquarters of the court marshals service, Moskovsky Komsomolets reported, citing law enforcement officials. The son, however, was not at work at the time of the attack, it said.
Kommersant, also citing investigators, said the attack might be revenge from militants angered by a recent series of law enforcement operations against them.
Averting another possible explosion, police found a bomb attached to the undercarriage of a police car parked in another part of Makhachkala at 8 a.m. Tuesday. “The explosive device found under the police car was neutralized,” police said in a statement carried by Itar-Tass.
Of those injured in Monday's attack, 41 have been hospitalized, including one child, and six were in serious condition, according to the Health and Social Development Ministry.
An Emergency Situations Ministry plane was sent to Makhachkala to transfer three seriously injured victims to intensive care units in Moscow and Nizhny Novgorod .
The Il-76 plane with a team of doctors from the National Center for Disaster Medicine flew out from Ramenskoye airfield in the Moscow region and arrived in Dagestan's capital at 10:25 p.m. Monday.
The aircraft was equipped with five intensive care units and can transport up to 20 patients. The medical team included two doctors, an anesthesiologist, a trauma surgeon and two nurses, Interfax reported.