The only clue to Thursday's bloody attack was a menacing message written on cardboard and left at the scene threatening politicians fighting organised crime, police said.
At least four men in two cars forced the bus to the side of the road and opened fire on it with assault rifles.
"Sixteen people died at the scene of the attack, and seven more died in hospital," police official Wilmer Torres said. "As well, there are at least 10 wounded."
Police have arrested a member of the notorious Mara Salvatrucha gang in connection with the killings, Torres said.