"The trailer-truck was on the shoulder of the highway at kilometer 135 (mile 84) ... when the bus hit the back of the trailer, which caused this regrettable accident," Governor Javier Duarte said.
"In addition to crashing, the bus caught fire, which makes the investigative work to identify bodies more difficult," he told Milenio television.
Federal police concluded that the truck was badly parked, the state government said. A state prosecutor was sent to investigate the crash.
At least 36 people were confirmed dead, said government spokesman Alberto Silva Ramos. Duarte had earlier reported 25 deaths.
Those injured have been hospitalised and are in stable condition, officials said.
The accident took place between 1.30am and 2.00am local time (0630 GMT and 0700 GMT) as the bus took passengers, mostly shopkeepers, from the southeastern Tabasco state city of Villahermosa to Mexico City.
The tragedy occurred as Mexicans are set to begin Holy Week in the predominantly Roman Catholic country.
President Enrique Pena Nieto expressed his "deepest condolences" to the families of the victims via Twitter.