Updated

SAN ANTONIO -- U.S. Border Patrol agents fired gunshots into Mexico after coming under attack during a half-ton drug bust and giving chase to a truck along the Rio Grande, U.S. authorities said Monday.

No Border Patrol agents were hurt during the "fire fight" early Saturday in Mission, agency spokeswoman Rosalinda Huey said. She did not say whether Border Patrol gunfire hit anyone, citing the ongoing investigation.

"The firing they received came from the Mexican side," Huey said.

Huey said several Border Patrol agents, at least some of whom were patrolling in boats, were seizing a half-ton of marijuana when they came under gunfire. Federal officials said the shots from Mexico began when a truck that was being chased by another group of Border Patrol agents entered the area.

FBI special agent Jorge Cisneros said the truck, which was on the U.S. side, appeared to be connected to the drug seizure. He said the gunfire from Mexico was a "direct result" of Border Patrol agents doing their jobs.

Cisneros described the shootout as brief.

"We're obviously concerned with what happened, that they would be shooting from the Mexico side to us," Cisneros said.

Federal officials did not release how many agents were involved, how many shots were fired or the number of shooters on the Mexico side. Cisneros said the FBI was working with Mexico authorities, including the Mexican military and the Tamaulipas state police, to determine what happened.

It was at least the second time in three months that Border Patrol agents in Texas have fired into Mexico. In June, a Border Patrol agent fatally shot a 15-year-old Mexican boy after authorities say a group trying to illegally enter Texas threw rocks at officers near downtown El Paso.

Reports of bullets whizzing across the border from Mexico also are on the rise. At least eight bullets have been fired into El Paso in the last few weeks from the rising violence in Ciudad Juarez, Mexico, where drug violence has killed more than 4,000 people since 2009, making it one of the deadliest cities in the world.

Cisneros said he can recall a handful of times in the last few years that gunfire from Mexico has crossed over the border. He said Border Patrol agents "have always been very good about not shooting back unless there is a life-threatening situation."

Huey who would not say whether the agents involved in the shooting still were on patrol. She said agents are authorized to fire their weapons any time they feel lives are at risk, even into Mexico.

"As long as our agents feel their life is in danger, they are allowed lethal (force)," she said.