"It is not clear how the grenade exploded," he said, but added that it was not uncommon for tribesmen to carry guns and hand grenades.
Mir Aslam Khan, a tribesman injured in the blast, confirmed that some of the passengers were armed.
"We were travelling and suddenly there was blast," he said.
The deaths came after 12 children were killed after a bomb planted in a football exploded earlier on Saturday in Lower Dir, 260km east of Miranshah.
The area is in the Malakand division of North West Frontier Province (NWFP) where the Pakistani government recently allowed a strict form of sharia (Islamic law) to be implemented as part of a deal to gain tribal support against Taliban fighters operating in the Swat valley region.
Taliban and al-Qaeda fighters have gained increased prominence in NWFP and North Waziristan this year, from where they have launched attacks both within Pakistan and neighbouring Afghanistan.
The Pakistan government is under pressure to gain control of the situation, seen by western nations as being crucial to securing Pakistan and improving conditions in Afghanistan, where US and Nato-led forces have been operating since the US-led invasion of late 2001.