DONNA, Texas —  Two Border Patrol agents investigating bundles of marijuana left along the Rio Grande exchanged gunfire with unseen assailants shooting from the Mexican side of the river, officials said.

The agents were unharmed Tuesday, and no one was believed injured on the Mexican side, Customs and Border Protection spokesman Oscar Saldana said.

It was the second time this year agents were fired at near that section of the river, which is less than 150 feet wide at that point.

The agents had spotted an inflatable raft near the U.S. side of the river near Donna, a small town near the southern tip of Texas, and found four bundles of marijuana totaling 305 pounds on the river bank, Saldana said.

"They went in to take a closer look. At that time the agents came under fire," Saldana said. "They immediately took cover and responded by firing as well."

The raft, carrying more marijuana, apparently returned to the Mexican shore.

Shots also were fired at agents in the area on Jan. 17.

"We anticipate more incidents like this one," Saldana said. "The smugglers feel frustrated. They're starting to take more and more drastic measures."