A spike in violence in Iraq continued Thursday with a double bombing in Kirkuk. The blasts killed at least 27 police officers and wounded another 70 in the oil-drilling center. Both bombs were in cars; a first blast injured some officers and drew a crowd, setting up a second bomb that injured many more and caused fatalities. As a boundary town between Kurdish, Arab, and Turkmen populations, Kirkuk is often the site of tension and violence, but this is the latest in a string of incidents, including a blast that killed 16 officers in Hillah and attempts at jailbreaks elsewhere. No one has claimed responsibility, but the chief of police in Kirkuk suggested al Qaeda was to blame.