(CNN) -- A U.S. Border Patrol agent has shot and killed an apparently undocumented migrant who allegedly threw rocks at the agent, striking his face, on the California-Mexico border near San Diego, authorities said.
A social justice advocacy group on Wednesday criticized the Border Patrol's use of deadly force in the Tuesday incident, saying there's a disturbing trend of Border Patrol "killings of rock throwers, some of whom were teenagers and some whom were U.S. citizens" on the U.S.-Mexico border, said Andrea Guerrero, executive director of Alliance San Diego.
The Border Patrol declined to comment directly on this week's incident, which is under investigation by local authorities, but in the past, the agency has highlighted how agents are seriously injured in rock-throwing incidents and resort to lethal force to protect themselves.
Guerrero said, however, some police experts advise using nonlethal force such as pepper-ball guns to respond to rock throwings.
"Border agents should adhere to best policing practices and only use lethal force as a last resort, not the only resort," Guerrero said.
In the San Diego sector on the U.S.-Mexico border, the Border Patrol has recorded more than 400 assaults, including rock throwings, on agents since 2010. The numbers have fluctuated in recent years, from 130 assaults in 2010, 77 in 2011, 133 in 2012 to 73 in 2013, according to the agency's statistics.
Names of the agent and the apparently undocumented migrant in this week's deadly encounter haven't been released by authorities.
Rock-throwing incidents have been controversial on the U.S.-Mexico border, particularly when Mexicans have thrown projectiles from the Mexico side of the international line toward Border Patrol agents on the U.S. side.
In some cases, U.S. Border Patrol agents have fired their guns at the rock throwers on the Mexico side of the border, killing the rock thrower. Mexican officials have condemned the Border Patrol's use of lethal force as excessive.
The U.S. Border Patrol said the incident occurred in a rugged section of the Otay Mountains near San Diego where the agent was trying to arrest several immigrants who crossed the border illegally.
"During the attack the agent was struck in the head with a rock," the Border Patrol said in a statement. "Fearing for his life the agent discharged his service issued weapon resulting in the death of one assailant."
Two other undocumented migrants were arrested and in custody, the agency said.
The San Diego County Sheriff's Department, which is investigating the killing, said in its account that two Border Patrol agents were trying to detain a group of suspected undocumented migrants in rugged terrain in the mountain's White Cross area about four mile east of the Otay Mesa border crossing.
"The two agents were out of sight of each other when one of the agents was struck in the face by a rock thrown by one of the suspects," Lt. Glenn Giannantonio said in a statement. "Fearing for his safety, the agent fired his duty pistol at the man, striking him. The man was pronounced dead at the scene."
The border patrol agent who fired his service weapon suffered minor injuries, the sheriff's homicide unit said.