The mayor of a Mexican border town said U.S. Border Patrol agents shot two city workers with pepperball guns early Wednesday while the workers picked up trash.

A pepperball gun launches hard plastic spheres that burst on impact and are filled with enough pepper derivatives to irritate the eyes, nose and throat. It is one of many so-called "sub-lethal" weapons that police and military units have acquired for crowd control problems.

Ruben Espino, mayor of San Luis Rio Colorado, called the shooting "aggressive actions" and complained about the incident to Gov. Janet Napolitano, who was meeting with him Thursday in Mexico. Espino said in a statement he will ask that the agents be disciplined.

Border Patrol spokesman Lloyd Easterling said the agency is investigating.

"As we should, we take all of these complaints seriously," Easterling said. "We're trying to figure out details and find out what's going on."

The two city workers were on a trash detail near the U.S.-Mexico border crossing early Wednesday when two U.S. agents asked them to identify themselves, even though they were on the Mexican side of the fence, according to the La Prensa newspaper in San Luis Rio Colorado.

The city identified the workers as Francisco Estrada Estrada and Gabriel Ramirez Garcia, who were in city uniform and using vehicles with a city logo at the time.

A city press release said one agent continued to fire at the two workers even after a Mexican police unit arrived.

La Prensa published photos of Estrada with welts on his back. He said he was hit at least four times.