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The boy’s mother said he had gone to eat with his brother, who handles luggage at a border customs office. While there, he met up with a group of friends and they decided to hang out by the river, she said.

“That was his mistake, to have gone to the river,” she said in an interview with Mexico’s Milenio TV. “That’s why they killed him.”

Mexico’s Foreign Relations Department said its records indicate the number of Mexicans killed or wounded by U.S. immigration authorities rose from five in 2008 to 12 in 2009 to 17 so far this year, which is not half over.

T.J. Bonner, president of the union representing Border Patrol agents, said rock throwing aimed at Border Patrol agents is common and capable of causing serious injury.

“It is a deadly force encounter, one that justifies the use of deadly force,” Mr. Bonner said.