A roadside bomb struck a vehicle carrying security forces in a former stronghold of local militants in northwest Pakistan on Wednesday, killing four, police said.

The attack happened in Bajur, a district in Khyber Pakhtunkhwa province on Pakistan’s border with Afghanistan. The area served as a base for the Pakistani Taliban until a few years ago, when the army said it cleared the region of insurgents. But the violence has continued there.

There was no immediate claim of responsibility for the latest attack.

Senior police officer Abdul Samad Khan said two police officers and two soldiers were killed in the attack. He said troops launched a search operation in the region to find those who orchestrated the attack.

Khan refused to speculate on who could be behind the attack.

But suspicion fell of Pakistan’s own Taliban who have been emboldened by the return to power of the Taliban in Afghanistan, where thousands of Pakistani militants are still believed to be hiding.

Pakistan and Afghanistan share a 2,400-kilometer (1,500-mile) internationally recognized border known as the Durand Line, which was drawn in the 19th century when the British dominated South Asia. Kabul has never recognized the boundary.

Before the Taliban came to power in Afghanistan, Pakistan and Afghanistan often accused each other of turning a blind eye to militants operating along the porous frontier.