The explosion happened on September 27 in the Charsadda district of Khyber Pakhtunkhwa Province and targeted provincial officials who were traveling home from work.
"Preliminary reports suggest that a remote-controlled improvised explosive device was planted in the rear section of the bus and that this was the private bus of the Civil Secretariat," the inspector-general of the province's police force, Nasir Durrani, told reporters in Peshawar.
Police suspect the bomb had some 12 to 15 kilograms of explosives and was packed with ball bearings.
A local resident, Fazal Rahman, witnessed the blast.
"I was buying medicine from the store where there was a huge explosion and we dropped to the ground. It was a huge bang and [we saw] everything hanging on the trees, such as the seats [from the bus], while the people were scattered on the road," Rahman said.
"Everything was scattered. Everything was very much damaged. Parts of bodies were there and some parts were missing."
Another witness, who gave his name only as Masood, said the explosion was so powerful that it injured bystanders.
"Around 10 people from my village were injured. They were not on the bus. They were sitting in a roadside shop and got injured when the bus exploded," Masood said.
"The situation was very bad. It was a very powerful explosion. The bang was heard in places 5 kilometers away."
So far, no one has claimed responsibility for the blast.
The area where the explosion happened is a known haven for Taliban and Al-Qaeda-linked militants.