Islamabad, Pakistan (CNN) -- Local officials in a northwest Pakistan province recovered the bodies of four men who apparently had been killed for allegedly collaborating with the United States.
The men had pieces of paper on them that warned others of the same fate if they cooperated with American officials, officials told CNN.
The two -- a local official and intelligence official who did not want their names used because they are not authorized to speak publicly on the matter -- said the bodies were found in the North Waziristan province.
It was not immediately clear how the men died or what precisely they were accused of.
Anger has run high in the area since a drone strike Thursday in which more than 40 people were killed.
The attack unleashed tribal fury, with regional elders vowing revenge on the United States.
Based on a CNN count, Thursday's suspected drone strike was the 18th of the year, compared with 111 in all of 2010.
U.S. officials rarely acknowledge publicly the CIA's secret drone program in Pakistan. Privately, U.S. officials say the covert strikes are legal and an effective tactic in the fight against extremists.