At least six people were killed when a cargo train derailed in a remote area of southern Mexico.
The train is nicknamed "La Bestia" or "The Beast" and known as a way would-be migrants hitch lifts toward the US border.
Ambulances could not reach the accident scene in Huimanguillo in the southern state of Tabasco due to the difficult terrain.
Images from the scene showed freight cars upturned and wheels separated from their base.
Officials said eight of the 12 cars overturned and a Honduran man was among the dead.
"This train carries a lot of (illegal) migrants from Central America," Cesar Burelo, emergency services director in Tabasco, told local television.
"It could be that as the structure of the train is removed ... more bodies could appear."
Mr Burelo said it would be difficult to determine the total number of people aboard the train at the time of the pre-dawn accident.
"It's very likely that (surviving travellers) have left the scene."
A spokesman for Tabasco state government said the death toll had risen to six, with 22 injured.
He said 16 people were being treated in hospitals in nearby Choapas, Veracruz.
There were different reports of numbers injured. Luis Felipe Puente, national emergency service coordinator, said on Twitter that 35 people were injured, 16 seriously.
Migrants often jump aboard "La Bestia" under cover of darkness. At times, several hundred migrants have been found on the train, either in freight cars or sitting on top.
Mexico's foreign ministry said it was helping Central American embassies whose citizens were affected by the crash.