"The survivors told us that about four miles before the scene of the accident, the driver said, 'We lost our brakes, I'm going to try to brake with the motor,"' Giron said.
But the attempt to downshift was apparently unsuccessful, damaging the gearbox and leaving the bus out of control, Giron said. Initial investigations suggest the bus had already flipped on its side and slid about 200 yards along the highway before rolling down the slope into a gully.
The bus driver was among the seven who died of their injuries at local hospitals.
The bus, which had an official capacity of 40 passengers, was carrying about 70 people when it was crushed by the impact of the crash.
The wreck occurred near the city of Cuilapa, about 25 miles east of Guatemala City. Forty-five people were found dead at the scene; the 17 remaining injured are being treated at nearby hospitals.
Guatemalan buses are frequently overloaded and must navigate rudimentary roads.