MIR ALI: The bullet-riddled bodies of four Pakistani tribesmen, killed by the Taliban for allegedly spying for the United States, were found Sunday in a semiautonomous tribal region near the Afghan border, witnesses and officials said.

Hours later, a bomb exploded in Pakistan's southwestern province of Baluchistan, where authorities have been fighting an ethnic insurgency. The blast killed at least three people and injured 12 more.

The bomb — planted on a bicycle in Quetta, the capital of Baluchistan — exploded as a police van drove past. It killed two policemen and one civilian, hospital officials said. Hamid Shakil, a senior police official, said it was apparently triggered by remote control. He blamed the attack on Baluchi nationalists — a militant movement separate from the Taliban insurgency that has been fighting the central Pakistani government for years.

Meanwhile, in the tribal region of North Waziristan, officials said the four dead men were kidnapped by the Taliban about 10 days ago.

Gul Akber Khan, who lives in the village of Srakhula, just outside of Mir Ali, said he heard gunshots in the middle of the night. When he went to the mosque for morning prayers a few hours later, he found the bodies dumped along the road into Miran Shah, the main city in North Waziristan.

Intelligence officials, speaking on condition of anonymity because they were not authorized to talk to the media, said a note was found by one of the corpses. It warned, in Pashto: "Spies are spies, and they will come to the same fate as these men. ... Do not spy for America."

The tribal regions along the Pakistan-Afghanistan border, strongholds for al-Qaida and Taliban fighters, have seen bloody fighting and regular attacks by American drone aircraft as the Pakistani and U.S. governments try to defeat the Islamist militants.