A Pakistani official said Sunday that militants ambushed a seven-member medical team vaccinating against polio in a remote tribal region on the border with Afghanistan, killing two workers and seriously injuring another two.

An official in Pakistan’s restive Mohmand Agency, Younus Khan, said Sunday the attack took place late Saturday and two of the workers were still missing. He did not suspect they had been kidnapped, but rather had fled the scene seeking safety.

Polio workers have come under relentless attacks by militants since it was revealed that attempts to identify Osama Bin Laden had involved using polio vaccinations as a ruse to enter his compound in Pakistan’s garrison city of Abbottabad where he was found and killed in 2011.

Pakistan is one of the few countries still battling polio.