According to preliminary reports from U.S. officials, Border Patrol agents on bicycles were pelted with rocks while trying to apprehend migrants trying to cross illegally from Ciudad Juarez into El Paso near a downtown bridge, an area known for drug and human trafficking, and monitored by video cameras and constant patrols.

U.S. officials said an unidentified Border Patrol agent was defending himself as the officers came under attack. The teenager, Sergio Adrián Hernández Güereca, 15, was shot in the head Monday, according to Mexican officials. Both governments and the FBI are trying to determine whether Hernández was standing on U.S. or Mexican soil at the time of the shooting, whether he was throwing rocks, and whether he posed a credible threat.

Police in the state of Chihuahua said the teenager died on the Mexican side of the border. Investigators retrieved a .40-caliber bullet casing.

U.S. officials said the Border Patrol agent has been placed on administrative leave, and a spokesman for the Department of Homeland Security issued a statement saying the U.S. government "regrets the loss of life and awaits the results of a complete investigation into this incident."

Mexican President Felipe Calderón, a close U.S. ally who is relying on help from Washington in the drug war raging just south of the border, said Mexico "will use all resources available to protect the rights of Mexican migrants." In a statement, he said his government "reiterates its rejection to the disproportionate use of force on the part of U.S. authorities on the border with Mexico."

José Reyes Baeza, the governor of Chihuahua, said U.S. agents were too quick to shoot at Mexicans, and he blamed "the xenophobic and racist conduct" on the passage of a new immigration law in Arizona that lets city and state police detain people who are in the United States illegally.

Tempers are running high along the U.S.-Mexico border because of violence from the fight against the drug cartels; the Arizona immigration law; and an incident two weeks ago in which Anastasio Hernández, 32, from Mexico, died after a Customs and Border Protection officer shocked him with a stun gun at the San Ysidro border crossing, which separates San Diego and Tijuana.

In Ciudad Juarez, a city beset by drug violence, the slain teenager's father told the El Paso Times that his son did not take drugs, was not in a gang and was not trying to cross into the United States. He said his son was just hanging out, as many teenagers do, along the trickle of water that is the Rio Grande, which separates the two downtown areas.

"He shouldn't have gotten close to those cowards -- what this dog did -- shoot into Mexico," Jesús Hernández told the El Paso Times.

U.S. Border Patrol agents are routinely pelted with rocks and bottles while chasing and apprehending illegal crossers, and have sometimes been shot at from Mexico. Although most illegal migrants are trying to enter the United States to find work, the borderlands are constantly crisscrossed by drug, human and weapons traffickers, many of them armed.

**Staff writer Jerry Markon in Washington contributed to this report.