Border Patrol agents were fired upon early Friday morning, the agency reported

Agents from the Rio Grande City Station Marine Unit were patrolling near the town of Fronton when the shots came from the Mexican side of the Rio Grande, according to the agency.

A U.S. Customs and Border Patrol boat heads south in the Rio Grande River between Hidalgo and Los Ebanos in Hidalgo County on routine patrol in 2007.

U.S. Border Patrol agents in the Rio Grande Valley were fired at early Friday morning while on patrol in Starr County, the agency said in a statement.

Agents from the Rio Grande City Station Marine Unit were patrolling near the town of Fronton when the shots came from the Mexican side of the Rio Grande, according to the agency. Fronton is about 20 miles west of Rio Grande City.

“Agents saw four subjects with automatic weapons who shot over 50 rounds at them,” an agency spokesperson said in a statement. “The boat was hit several times but no one on board was injured.”

The Border Patrol said the shooting is under investigation.