Border Patrol officials say as many as 20 shots fired at agents near Brownsville, Texas, originated from Mexico in the most recent of a series of incidents involving gunplay and other violence directed at Americans from south of the Rio Grande River.

U.S. officials said last weekend’s gunfire did not appear to be a random action and that the attacker – who was not identified in reports – was most likely a trained shooter, KRGV-TV, ABC’s Brownsville affiliate, said.

Jose Rodriguez, a spokesman for the Border Patrol in McAllen, Texas, told Reuters, “Shots were fired, no one was injured, and the FBI have taken the case over.”

Authorities said as many as five shots struck a Border Patrol boat as it patrolled the area.

In a separate shooting days earlier, U.S. officials said as many as 10 shots were fired at agents patrolling near the Veteran’s International Bridge at Los Tomates. One Border Patrol vehicle was struck but no agents were injured in that shooting, either.

U.S. officials believe drug and people smugglers are responsible for the increased acts of violence, perhaps in retaliation for increased border security that has led to more arrests.

Meanwhile, a week ago a Border Patrol agent near San Diego shot and killed Guillermo Mart?nez, “a known people smuggler who had been detained 11 times prior,” agency spokesman Raul Martinez told The Associated Press.

The shooting death has set off a storm of protest in Mexico, with officials there promising an investigation amid charges U.S. immigration policy sanctions the improper use of force.

Border Patrol officials say the shooting was justified, however, because the agent feared for his life and retaliated only after being attacked with rocks.

Mart?nez slipped back across the border after he was shot, where he died at a hospital in Tijuana the next day.

Related special offer:

Could Mexico retake the southwestern United States?

Previous stories:

Border Patrol fears conflict with Mexican military

Increasing alarm over border security

Border Patrol encounters Mexican soldiers

Border accident or bounty hunting?

Mexicans declare border war

Pressing Reno for Border Patrol body armor

Make a gun for the border

Note: Read our discussion guidelines before commenting.