At least 18 people died in western Mexico when a passenger bus plunged off a highway into a ravine early on Thursday, state officials said, adding the passengers were mostly foreigners and some were heading for the U.S. border, Reuters reported.

En route to the northern border town of Tijuana, the bus had been carrying around 42 passengers, including citizens from India, Dominican Republic and African nations.

The bus driver has been detained, the Nayarit state government said in a statement, with authorities suspecting him of speeding round a bend in the road.

Officials said they were still working to identify those who had died on Thursday afternoon.

Around 20 people have been taken to hospital to treat injuries, with one woman’s condition described as “delicate,” the state government said.

The bus, part of the Elite passenger line, crashed near Barranca Blanca on the highway outside state capital Tepic, officials said.

Rescue has been “extremely difficult”, said Jorge Benito Rodriguez, security and civil protection secretary for Nayarit, as the ravine was some 40 meters (131 ft) deep.

According to Reuters neither the bus company nor Mexico’s migration institute immediately responded to requests for comment.

Last month, a bus crash in the southern state of Oaxaca left 29 people dead, and in February, a bus carrying migrants from South and Central America crashed in central Mexico, killing 17, read the report.