Mexico City ? A tanker truck hauling toxic ammonium chloride slammed into a passenger bus in northwest Mexico, killing 38 people as both vehicles plunged down an embankment, police said Wednesday.

The bus flipped onto its roof in Tuesday night’s crash and the tanker spilled it load of 25 tons of ammonium chloride, officials said.

Most of the victims had skin damage caused by chemical burns and authorities were trying to determine whether the ammonium chloride contributed to their deaths.

“The majority … had apparently ingested ammonia, as well as having various contusions,” Jesus Martin Robles, chief regional investigator for the Sinaloa state Attorney General’s Office, said a written report.

The driver of the tanker, who was among the dead, lost control of his vehicle on a curve – possibly as a result of brake problems – and crashed into the rear of the bus. A police report said both vehicles then plunged down slopes on the side of the highway.