An explosion ripped through a police truck on Wednesday killing at least six people in Pakistan’s southwestern city of Quetta, police told Reuters.

The truck was carrying police recruits, four of whom were killed, along with two passers-by on the city’s eastern outskirts, Quetta Police Chief Abdur Razzaq Cheema said.

“We can’t say whether it is a suicide blast,” he added. “Our teams are at the scene and investigating.”