(CNN) — The death toll from a cargo train derailment in Mexico climbed to six on Monday as authorities continued searching the scene of the wreck for victims.
The cargo train was hauling 55 tons of scrap metal, but hundreds of stowaway migrants were also on board, and officials feared the number of dead could rise as they combed through the wreckage.
All six people killed in Sunday's derailment were Honduran, Mexico's National Migration Institute said. Fifteen Hondurans and Guatemalans were hospitalized after the derailment. By Monday, five remained hospitalized in serious condition.
Mexico no longer has a nationwide passenger rail system, but thousands of Central American migrants hitch rides on freight trains heading north toward the United States.
On the train, nicknamed "The Beast" and "the train of death," they often huddle on rooftops and cram into spare spaces between cars.
Honduran migrant Saul Portillo Sanchez told CNN affiliate FOROtv he was riding on the train, watching over a woman who was sleeping, when he felt the car start to rock.
"It was horrible. ... I was going to jump but there wasn't time. I fell as the train car tipped," he said.
Eight cars overturned in the early-morning derailment, which authorities said occurred when the train was traveling about 10 kilometers per hour (6 miles per hour) through the municipality of Huimanguillo in Mexico's Tabasco state.
The train's engine and first car stayed on the tracks, allowing authorities to use them to transport injured victims to a regional hospital, officials said.
At least 59 migrants involved in the crash received support at a shelter in neighboring Veracruz state, the Mexican office of the United Nations High Commission for Human Rights said.
Mexican authorities have said they're investigating what caused the derailment.
On Sunday, Tabasco's state government said heavy rains in the area may have destabilized the ground around the tracks. Another hypothesis is that the train was overloaded, a Facebook post by a state government spokesman said, though officials later said that the train was going at a normal speed and carrying a normal load.
Some local media reports have suggested that portions of the track may have been removed, but CNN has not confirmed those reports.