Federal police intercepted 104 undocumented Guatemalan immigrants in the central state of Queretaro, Mexico's Public Safety Secretariat, or SSP, said Friday.

The migrants were detected during a routine inspection of a northbound bus and 13 people accompanying the Guatemalans were arrested as suspected migrant traffickers, the SSP said in a statement.

The migrants told police they each paid the traffickers about $1,000 for the trip from Guatemala to northern Mexico with a promise to pay another $3,000 once they made it across the border into the United States.

The Guatemalans were turned over to Mexican immigration authorities for repatriation, the SSP said.

An estimated 300,000 Central Americans undertake the journey across Mexico each year on their way to the United States, walking part of the way or riding aboard freight trains, buses and cargo trucks.

The trek is fraught with danger, with criminals and corrupt Mexican officials preying on the migrants.

Last year, about 20,000 Central Americans were kidnapped by organized crime groups, which extorted money from them or forced them to join their gangs, according to a report from Mexico's National Human Rights Commission.