Islamabad, Pakistan (CNN) -- The bullet-riddled bodies of six people were found Sunday in Pakistan's tribal region, four of them near a letter that accused them of spying for the United States, two local government officials told CNN.
Four of the bodies were found in front of a bus stop near the village of Mir Ali in North Waziristan, the officials said. The other two were found just outside the town of Miranshah, also in North Waziristan.
The letter was found near the four bodies. The officials said they did not know the exact wording of the letter, but said it was written in Pashto and warned that this would be the fate of those who spy for the United States.
There have been at least 21 U.S. drone strikes this year in Pakistan's tribal region, according to a CNN tally. The most recent, on Sunday night, killed five militants, according to officials. All of the strikes have hit locations in North Waziristan, where the bodies were found, or near the border with South Waziristan. The Taliban have killed dozens of people in the tribal region for allegedly providing information to the United States about militant leaders' whereabouts.
Earlier Sunday, a bomb aimed at a police official in southwest Pakistan killed three people.
The official, Nazir Kurd, was not in the vehicle that the attackers targeted in Quetta, police said.
But two police officials were among the dead. Eighteen others were hurt.
The bomb was placed on a motorcycle and remotely detonated, said police official Hamid Shakeel.
Quetta is the capital of the Balochistan province. Located near the Afghan border, it is the largest province in Pakistan but one of its least populated.
The blast is the latest in a series of attacks that have shaken Pakistan in recent months. Officials blame the violence on Islamic militants who have vowed retaliation against an army offensive to rout them from their haven along the country's border with Afghanistan.
Journalist Nazar Ul Islam contributed to this report for CNN.