Seven charity employees in Pakistan were gunned down Tuesday in the latest assault apparently targeting an anti-polio campaign in the country, the Associated Press reports.

Men on motorcycles sprayed gunfire into a van carrying five teachers and two health workers from Support With Working Solution, a community center in Khyber Pakhtunkhwa province whose activities included vaccinating children against polio. Six of the victims were women.

The center’s director linked the attack to opposition to the polio effort among militant groups, who accuse health workers of spying for the United States and harming Pakistani children. Nine people active in the vaccine campaign were killed in multiple incidents last month.