SAN YSIDRO, Calif. -- Investigators sought Wednesday to piece together what prompted a Border Patrol shooting that wounded a young Mexican national during a late-night confrontation at the international line in San Ysidro.

The incident began unfolding about 10 p.m. Tuesday, when an officer with the federal agency on patrol near the 300 block of Virginia Avenue spotted three or four people attempting to scale the south side of the border fence, according to San Diego police.

The agent fired a pepper-ball gun "in the direction" of the would-be illegal immigrants in an attempt to get them off the barrier, said Lt. Terry McManus of the SDPD Homicide Unit, which investigates all officer-involved shootings in the city.

Moments later, stones "larger than softball-size" and other debris began landing near the agent as he sat in his patrol vehicle, McManus told reporters during an afternoon briefing.

"He believed he was being attacked -- he, in fact, was being attacked by individuals on the other side of the border," the lieutenant said. "They were throwing large rocks, boulders, and in a couple of instances, pieces of concrete block towards him."

The agent radioed for backup, and when other Border Patrol personnel showed up, the onslaught of projectiles was still raining down, he said.

About half-dozen officers then went through an access door in the border barrier and were confronted by "seven to 15 Mexican nationals ... in a group, many of them still armed with rocks," McManus said.

After discharging about 50 more pepper balls, the agents launched a tear-gas grenade toward the group, and the resulting cloud of caustic chemical vapors prompted most of the aggressors to flee to the south.

"Two of the suspects, however, remained and were still armed with rocks and directing their attention toward the Border Patrol agents," McManus told reporters.

Fearing for his safety and that of his colleagues, one of the officers fired two rounds from his service rifle, striking one of the Mexican nationals, a 23-year-old man, the lieutenant said.

"That suspect immediately went down," McManus said.

Moments later, Mexican emergency personnel arrived in the area and tended to the suspect at the scene, then took him to a Tijuana hospital.

The wounded man reportedly was in stable condition. His name was not available.

It was unclear if the man sustained two gunshot wounds or just one, the lieutenant said.

The 35-year-old agent who opened fire is a 10-year veteran of the agency, McManus said, adding that his name will be withheld until investigations in the case are complete.

Such attacks on Border Patrol agents are not uncommon, Tijuanapress.com reporter Vicente Calderon told a local TV station.

"These incidents (happen) quite often," he said.

The U.S. Border Patrol and the Mexican Consulate in San Diego did not return telephone calls seeking comment.