Mexican authorities found 103 unaccompanied minors mostly from Guatemala inside an abandoned truck trailer in the eastern Mexican state of Veracruz, the government said on Monday, in one of the biggest recent discoveries of migrant children traveling through Mexico.

In addition to the 103 children, authorities found 212 adults from Guatemala, Honduras, El Salvador and Ecuador in the trailer, the National Migration Institute (INM) said in a statement. Another 28 migrants traveling as families from Guatemala and El Salvador were also in the trailer, bringing the total number of passengers to 343.

Authorities discovered the trailer on a highway without a driver, INM said. It was outfitted with fans, a partially ventilated roof and a structure that created a second level inside the trailer.

A photo that INM distributed showed plastic bottles and other trash strewn across the floor of the long, narrow compartment.

The children will be put into custody of Veracruz’s family services system while the other migrants will be processed to determine their legal status in Mexico, INM said.

Earlier this year, Mexican authorities found 57 unaccompanied minors from Guatemala at a checkpoint near the U.S.-Mexico border, and 20 other unaccompanied minors in a group of mostly Central Americans in the southern state of Chiapas.