SAMARRA, Iraq (Reuters) - A suicide bomber blew up a car outside a shelter housing police officers in the northern Iraqi city of Samarra on Monday, killing 13 policemen and wounding 25, police and hospital sources said. The victims were members of an elite special weapons squad sent to Samarra to protect shrines during a recent Shia religious event, a police source said. The attacker stopped the car in front of a shelter where a SWAT team from southern Maysan province was staying. Samarra, 100 km (60 miles) north of Baghdad, was struck by another suicide bombing that targeted Shia pilgrims nine days ago. The bomber detonated an explosive vest at a crowded bus depot just outside the city, killed at least 48 people and wounding 80. On Feb. 10 a suicide car bomber attacked a group of pilgrims near the town of Dujail as they headed to Samarra for the commemoration, killing eight and wounding 30. Overall violence has dropped sharply in Iraq since the height of sectarian violence in 2006-07 but bombings and other attacks still occur daily. Iraq has seen an average of about 12 attacks a day in recent months, US military officials said recently.