MEXICO CITY (AP) — Authorities in Mexico said Tuesday that 20 migrants were caught at a highway checkpoint using falsified paperwork with letterheads from the U.N. refugee agency, UNCHR.

The migrants were found aboard passenger buses at a checkpoint in the northern border state of Nuevo Leon.

When asked for documents, they displayed letters supposedly from the UNCHR stating they were refugees or had requested refugee status, and should be allowed to travel to cities in northern Mexico.

Mexico’s National Immigration Institute said Tuesday that U.N. officials confirmed the documents were false and have filed a complaint in the case.

The Institute said some of the migrants said that smugglers had given them the documents and promised they were a “safe pass” to the U.S. border.

The Institute said those detained included migrants from Honduras and El Salvador.

In recent months, migrant traffickers have become more brazen, and are increasingly using buses to smuggle migrants. Following a crackdown in 2014 on buses and trains, smugglers had mostly resorted to hiding migrants in the freight containers of trucks.