Officials said the man did not appear to be targeting Trump or others involved in the trial
The assault occurred late on Friday in the town of Mastung, around 40 km south of Quetta.
"The armed men were wearing the uniforms of the security forces," Sarfaraz Bugti told Reuters.
The bodies of 19 passengers had been found so far, he said.
Security officials said around 25 passengers were taken off two buses, and an operation began to find them.
The bodies were discovered in nearby hills, the officials said.
The circumstances of their deaths could not immediately be established, and the motives of the assailants was unclear.