A doctor at the main hospital in Bannu said the bodies of nine people killed in the attack were taken there, but a resident said the toll was higher because relatives had kept some of the bodies.
Mohammed described the attack as "terrorist activity", but said it was too early to say more about who was behind it.
Bannu and other cities in the northwest have suffered a string of shootings and bombings blamed on Taliban fighters who have been expanding their influence from strongholds in the tribal belt along the Afghan border.
The attacks and a bloody army siege of Islamabad's radical Red Mosque earlier this month have raised concern about the "Talibanisation" of Pakistan's North West Frontier Province, prompting of Pervez Musharraf, the president, to send thousands of security forces to the area.
Dozens of soldiers have been killed in suicide attacks in North Waziristan, the tribal region closest to Bannu.
Taliban leader dead
However, the government scored a victory on Tuesday, when Abdullah Mehsud, a Taliban leader who was one of Pakistan's most-wanted men, killed himself with a hand grenade as security forces surrounded him in a house in Zhob, a town in Pakistan's Baluchistan province.
Intelligence officials said Mehsud was returning after more than a year spent leading Taliban campaigns against US and government troops in southern Afghanistan.
Mehsud was captured fighting with the Taliban in 2001 and imprisoned at Guantanamo Bay.
He was released in March 2004, but quickly took up arms again, leading fighters in South Waziristan and orchestrating the kidnapping later that year of two Chinese engineers, one of whom died in a rescue raid by Pakistani commandos.