The toll is expected to rise as several buildings collapsed from the force of the blast, according to police on Saturday.
Muhammad Farid, the police deputy superintendent, confirmed that 40 people were injured, including 24 policemen, after the attacker drove his explosive-laden car into their building in the Mardan area on the outskirts of Bannu town.
Bannu is the gateway to North Waziristan - a tribal region on the Afghan border believed to contain al-Qaeda and Taliban fighters.
Civilians were also caught in the blast.
Al Jazeera's Kamal Hyder, reporting from outside Islamabad, said that the Tehreek-e-Taliban group had claimed responsibility for the bombing.
"We are told that it was a vehicle-laden bomb. And the vehicle was packed with explosives," he said.
"[There is] a 10ft-deep crater at the site of that particular explosion but it appears that the vehicle was packed well before time and there are indications that this may have been a remote-controlled bomb ... because we have no confirmation of any person present inside the car.
"And it's not yet clear as to what was the intended target of that particular bombing."
The group has threatened to unleash bigger attacks on government targets to avenge the killing of Baitullah Mehsud, their leader, in a US drone attack.
Second attack
In Saturday's second attack, a car bomb went off in a commercial district in the main northwestern city of Peshawar, the capital of NWFP, killing four people and wounding dozens more.
Confirming the death toll, Shaukat Khan, a local police officer, explained that an attacker detonated the device outside a bank affiliated with the army in Peshawar.
Our correspondent said that while the attacks were deadly, they could have been much worse.
"This attack came at the end of a long weekend holiday," he said.
"Shops were not open and far fewer people were killed than would otherwise have been the case."
The car bombing came less than 24 hours after a US unmanned drone aircraft killed 12 suspected Afghan fighters, according to Pakistani intelligence officials.
The missile attack took place on Thursday in Dandy Darpa Khel village, near the town of Mir Ali, adjoining North Waziristan.
"All those killed were Afghans; men who had come from Logar," a villager said referring to an Afghan province south of the capital, Kabul.
Pakistan has officially condemned US drone attacks, saying they violate the country's sovereignty.