Police in southern Russia on Thursday hunted down and killed four suspects following the killing of two police officers.

The Investigative Committee said the National Guard in the city of Astrakhan found the suspects, who refused to surrender and fired back at officers. In the ensuing gunbattle early Thursday, all four gunmen were killed and three National Guard officers were wounded.

The shoot-out followed Tuesday’s attack, in which two of the alleged suspects shot two traffic police officers dead at point-blank range and seized their weapons.

On Monday, a suicide bombing on St. Petersburg’s subway killed 13 plus the bomber and wounded more than 50 people. There has been no claim of responsibility for the attack, but Russian trains and planes have been frequent targets of attacks, usually blamed on militants.

In a separate incident in another southern Russian city, Rostov-on-Don, a man was seriously injured when he inadvertently detonated an explosive device. Thursday’s explosion occurred when the man touched a flashlight hidden in a bag left near a school building, according to the National Guard.

Russian law enforcement agencies have launched a manhunt for those responsible.