Images after the attack on Saturday showed the structure had been reduced to rubble, with bricks, charred wreckage, and mangled vehicles scattered around the area.
Police official Sajjad Khan said in a statement on Sunday that the bodies of 12 officers had been recovered from the collapsed outpost and three personnel were found alive and rushed to hospital.
A police official who asked not to be identified said militants first rammed into the post with an explosive-filled car and then entered the premises and began firing on any remaining officers.
"Other law enforcement personnel were sent to help the police, but the terrorists ambushed them and caused some casualties," he said.
Police sources said the militants also used drones in the attack.
Ambulances from rescue agencies and civil hospitals rushed to the scene, with officials saying a state of emergency had been declared in government hospitals in Bannu.
A militant alliance known as the Ittehad-ul-Mujahideen claimed responsibility for the attack.
Militant attacks have the potential to reignite fighting along Pakistan's border with Afghanistan.
The worst fighting in years erupted between the allies-turned-foes in February, with Pakistani airstrikes inside Afghanistan that Islamabad said targeted militant strongholds.
Fighting has since eased, with occasional skirmishes breaking out along the border, but no official ceasefire has been brokered.
Islamabad blames Kabul for harbouring militants who use Afghan soil to plot attacks in Pakistan.
The Taliban has denied the allegations and said militancy in Pakistan is an internal problem.
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