Attackers last year firebombed Plan International's offices in Mansehra
Police in north-western Pakistan say they have found the bodies of three local female aid workers and their male driver, all with gunshot wounds.
They were kidnapped and killed after they left Mansehra town to inspect a school for possible funding by the American development agency, USAID.
Police said the women worked for a USAID partner organisation, the National Rural Support Programme.
Attacks on aid workers are not uncommon in Pakistan.
In February last year, British aid agency Plan International suspended its operations in Pakistan after three of its workers were killed in an attack on its office in Mansehra.
The BBC's M Ilyas Khan in Islamabad says Mansehra has until recently served as a base for militants operating in Afghanistan and Kashmir.
Police say the three women were kidnapped shortly after they left Mansehra around noon on Monday.
Their bodies were found in a thickly forested area in the nearby Shinkiari area in the evening, they said.
"The bodies of the women were inside their car, while that of the driver was lying on the ground beside the car," a local police official told reporters.
The attackers are yet to be identified.
The National Rural Support Programme has been engaged in educational projects in this backward mountainous region and employs female social workers to explore avenues where USAID funds can be utilised.
Militant groups and religious parties in Pakistan have often accused non-governmental organisations (NGOs) of spreading "vulgarity" in society.
This is because the NGOs mostly employ women workers and organise mixed social gatherings in line with their professed policy of gender equality, observers say.
Last year, Plan International said gunmen stormed into its office in Mansehra without warning, opening fire indiscriminately.
The attackers killed three people and wounded eight others. They set the premises on fire before fleeing in two Plan vehicles.
The children's charity had worked in the area for a decade and was active in providing aid following the 2005 earthquake in Pakistan.