Most weekends, Carolyn Powers takes her horse to Border Field State Park along the U.S.-Mexico border, riding past clusters of people on the other side of the 10-foot metal fence.

She's seen some on the Mexican side pick up rocks from the hilly terrain and shower U.S. Border Patrol agents as they pass by in their white and green Ford Broncos.

Tensions have dramatically increased on both sides of the fence following four shooting incidents in as many days involving illegal immigrants and U.S. officers.

None of this frightened Powers - until she became a target.

She and a friend were riding the trails Sunday when three women threw rocks at them from over the metal fence, causing her friend's horse to bolt.

"It's anti-American sentiment," Powers, 53, said. "It's not as easy as it used to be to get across the border. Maybe they're angry about that."

Within hours near the same spot, a Border Patrol agent fatally shot an illegal immigrant he said threatened him with a fist-size rock. It was the second fatality in two days.

Authorities on both sides wonder why there has been such a surge in border violence. They hope an investigation by the FBI will eventually yield some answers; no one is happy with the image of federal agents gunning down rock-toting immigrants.

"The last thing an agent wants to do is draw a weapon and use it. An agent would rather make apprehension or chase someone back across the border," said Bill Strassberger, spokesman for the federal Immigration and Naturalization Service, which oversees the Border Patrol.

"It's very unusual to have that many shootings, but it's also unusual that we've had agents attacked that many times," he said.

San Diego was long the preferred place to sneak across the 2,000-mile Mexican border until four years ago, when Operation Gatekeeper increased the number of Border Patrol agents from 800 to about 2,300.

The idea was to restrict the flow of illegal immigration at San Diego and push migration east to more dangerous routes in Imperial County. One result, Strassberger said, is that exasperation on the Mexican side has been building.

Last week, an undocumented immigrant was wounded by a Border Patrol agent in eastern San Diego County when he allegedly tried to run over the officer with his car. The 20-year-old man was shot in the chest.

Two days later, on Saturday morning in eastern San Diego County, an agent fired at an illegal immigrant who allegedly tried to run down the officer with his car. The immigrant, who was not hit by the gunfire, later crashed his car and was arrested.

Later that day, Oscar Abel Cordoba Velez, 23, was fatally shot after he allegedly picked up a rock and approached an agent at the San Ysidro border fence, about 15 miles south of San Diego. Mexican witnesses said the man wasn't menacing the officer.

Mexican Consulate Luis Herrera-Lasso, whose office is in San Diego, said U.S. authorities have enforced Operation Gatekeeper for three years without firing their weapons. He said frustration from illegal immigrants would have peaked before now.

"They don't want to have an incident of any kind. They only want to pass through," he said. "They know they have no way to win a fight with the Border Patrol."

Herrera-Lasso asked U.S. District Attorney Charles La Bella on Tuesday to review the Border Patrol's policies on when to fire weapons and to share with Mexican authorities the results of the federal investigation.

"After three years of not having any shootings, we have had four in four days," he said.

Mario Villarreal, a Border Patrol spokesman in San Diego, said the agents apparently fired in self-defense. He said it was a coincidence that four agents from four different divisions fired their weapons in as many days.

The Border Patrol has not identified the agents involved in the shootings.

Herrera-Lasso hopes an investigation can lead to answers.

"We are concerned over the apparent use of excessive force," he said. "The situation must be reviewed carefully so hopefully we can avoid more shootings."