A suicide car bomber killed six police officers in the capital of the Russian region of Dagestan on Wednesday. It was the deadliest such attack in months in a region where assassinations of police officers is a defining characteristic of the local militant Islamist movement. The bombing came five days after the police said they had killed a local militant leader. Officials said that the toll could have been far higher: the driver was stopped at a checkpoint just outside a traffic police station where, according to Russian news agencies, officers were mustering for roll call. The driver apparently realized that he could not get past the checkpoint and instead detonated the bomb as a truck carrying police officers passed by, the RIA news agency quoted Vladimir Markin, a spokesman for the Investigative Committee of the Russian Prosecutor's Office, as saying. About a dozen officers were wounded. A similar attack in August, in Ingushetia, another region of the restive, predominantly Muslim north Caucasus, killed 25 people in a police yard. Other attempts were made Wednesday to carry out attacks in the Dagestani capital, Makhachkala: grenades were thrown onto the lawn of a local official, and a bomb was placed under a railroad track. But the railway bomb was defused before it exploded, and the grenades caused no injuries. The attack on the police brought a swift response from President Dmitri Medvedev, who ordered the director of the Federal Security Service, Aleksandr Bortnikov, and the interior minister, Rashid G. Nurgaliyev, to investigate the attack and 'take all the measures necessary' to improve security. Medvedev traveled to Dagestan in June after the region's top police official was assassinated. On Jan. 1, Dagestani police reported that they had killed three militant leaders, including the militants' 'emir,' Umalat Magomedov, in a shootout. Among his belongings, authorities said, the police found a ledger of militants' finances from extorting local businesses and from donations from Saudi Arabia, Turkey and Azerbaijan. Violence had generally been in abeyance in the Caucasus for a few years, since the second Chechen war ended, but it has been surging back recently. On Wednesday, three police officers were wounded by an explosive device in Chechnya, and a bomb was found under a natural gas pipeline in Ingushetia. 