A US Border Patrol marine unit was attacked early Friday by gunmen who fired more than 50 rounds at the agents from the Mexican side of the Rio Grande, officials said.

None of the agents assigned to the Rio Grande City Station Marine Unit was injured while they were patrolling near Fronton, Texas, US Customs and Border Protection said in a news release.

“Agents saw four subjects with automatic weapons who shot over 50 rounds at them. The boat was hit several times but no one on board was injured,” the CBP said.

A Border Patrol spokesman couldn’t immediately say whether the agents returned fire. The incident remains under investigation.

The Rio Grande separates the US and Mexico in Texas. Border agents and Texas authorities patrol the river and sometimes rescue migrants trying to cross.

The territory across the river from Texas is dominated by violent gangs.

American authorities have forced thousands of asylum-seekers to wait in Mexico, citing capacity issues.

Meanwhile, agents assigned to the Rio Grande Valley Sector encountered two large groups of suspected illegal immigrants who turned themselves in, according to the CBP.

On Sunday, agents in McAllen, Texas, observed a large group of people congregating along the border wall near Hidalgo, Texas.

“Responding agents advised that the group of 100 consisted of family units, unaccompanied children and single adults from Honduras, El Salvador, Guatemala, Ecuador and Nicaragua,” the agency said.

On Wednesday, McAllen agents working in Los Ebanos, Texas, responded to a report of people walking north of the Rio Grande. Responding agents located the group of 146 suspected illegal immigrants similar to the earlier group, according to CBP.

With Post wires