A US Border Patrol boat patrolling the Rio Grande was shot at early Friday morning from the riverbank on the Mexican side, US Customs and Border Protection, or CPB, reported.

More than 50 rounds were fired and the boat was hit several times, but no one on board was injured, according to the agency .

Shots rang out while the agents were patrolling near Fronton, Texas, the Rio Grande City Station Marine Unit reported. Agents saw four people with automatic weapons on the Mexican riverbank, CPB reported.

The incident is under investigation.

The Rio Grande City station's area of responsibility encompasses about 1,220 square miles of Starr County, Texas, including 68 miles of international border, the agency's website explains.

Three ports of entry -- Rio Grande City, Roma and Falcon Dam -- are located in the territory. Directly across the Rio Grande are the Mexican cities of Ciudad Camargo, Miguel Aleman, Mier and Nuevo Guerrero, all located in Tamaulipas, one of Mexico's 31 states.