A police officer in eastern Afghanistan shot dead nine of his colleagues as they slept Friday morning and then fled in a government vehicle full of guns and ammunition, according to Afghan and American officials. The nine had been drugged earlier, an Afghan official said.

The incident, which took place in Paktika province, marks one of the deadliest cases of fratricide in Afghanistan this year. The apparent surge in such incidents — when Afghan soldiers and policemen target their American and Afghan colleagues — has raised concerns about the state of the war effort during a critical time, just as the Taliban’s yearly “spring offensive” has begun.

On Monday, also in Paktika province, a different Afghan police officer killed a U.S. soldier. Two British soldiers were also killed on Monday by an Afghan soldier in the southern province of Helmand.

Both assailants in the Paktika incidents are believed to have been members of the Afghan Local Police, a force of local recruits armed and trained to keep insurgents from gaining ground, authorities said. The ALP has recently been under fire for alleged human rights abuses, and some critics say the force amounts to little more than a smattering of militias. Still, U.S. and Afghan defense officials say the ALP is key to policing restive districts and gaining the trust of local populations.

Friday’s incident, which is under investigation by American and Afghan forces, ended with the suspect driving off in a white Ford Ranger filled with 10 AK-47s and 25 magazines, a U.S. official said. Afghan police brought in the suspect’s two brothers for questioning, said Mokhlis Afghan, a provincial spokesman.

In two additional incidents, one NATO service member died in a makeshift-bomb attack on Friday and another was killed by an insurgent attack Thursday, according to Western officials. Both incidents took place in southern Afghanistan.