ISTANBUL (AP) — A car bomb hit a police vehicle in Istanbul during the morning rush hour on Tuesday, killing 11 people and wounding 36, the fourth bombing to hit the city this year.

There was no immediate claim of responsibility, but Turkey has seen a recent increase in violence linked to Kurdish rebels or to the Islamic State group, which has found recruits and established cells in the country.

Speaking at the scene of the blast in Beyazit district, Istanbul Gov. Vasip Sahin said a bomb placed inside a car detonated as a police vehicle passed by. The dead were seven police officers and four civilians. At least three of the wounded were in serious condition.

Sahin declined to comment on who may be behind the attack, and authorities imposed a news blackout preventing media from reporting details of the probe in Turkey, citing concerns over security and police and forensic efforts to investigate the attack.