MEXICO CITY (AP) — Mexico’s immigration service said Tuesday a group of migrants threw rocks at its officers, injuring one severely.

The National Immigration Institute said the attack happened Monday along railroad tracks in the central state of Tlaxcala.

Migrants frequently hop freight cars or follow railroad tracks to reach the U.S. border, and Mexican authorities often raid rail lines to catch and return them to their home countries.

The institute said its officers approached a group of migrants walking along the tracks in the town Apizaco and the migrants reacted by throwing rocks. One officer was hit and was in serious condition at a local hospital with wounds to the face and skull, it said.

Attacks by migrants on immigration officers in Mexico are very rare, though there have been scuffles and shoving matches in the past.

The institute did not identify the migrants’ nationalities, or say if any had been detained. Most migrants detained in Mexico are from Central America.

Also Tuesday, a group of about 70 non-governmental organizations and migrants' rights groups published a letter asking Mexican officials not to allow the U.S. to reinstate the “remain in Mexico” policy of sending asylum seekers back across the border to wait for hearings on asylum claims.

The U.S. Supreme Court on Tuesday refused to block a lower court ruling ordering the administration of President Joe Biden to reinstate the Trump-era policy of forcing people to wait in Mexico.

Mexico is not legally obligated to receive returning migrants who are not Mexican citizens, and most of the asylum seekers are not.

But President Andrés Manuel López Obrado has had good relations with the U.S. government on immigration matters and has willingly cooperated in blocking migrant caravans and deporting migrants trying to reach the U.S. border. López Obrador allowed the U.S. to implement the first version of the remain in Mexico policy under President Donald Trump.

It’s not clear how many people will be affected by the Supreme Court ruling and how quickly. Under the lower court ruling, the administration must make a “good faith effort” to restart the program.

There also is nothing preventing the Biden administration from trying again to end the program, formally called Migrant Protection Protocols.

During Trump’s presidency, the policy required tens of thousands of migrants seeking asylum in the U.S. to turn back to Mexico. It was meant to discourage asylum seekers, but critics said it denied people the legal right to seek protection in the U.S. and forced them to wait in dangerous Mexican border cities.