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Mr. Aguilar was killed as he tried to lay spike strips on a highway near the Imperial Sand Dunes in California, 20 miles west of Yuma, Ariz., to stop at least two vehicles fleeing from Border Patrol agents and back into Mexico.

“Agent Aguilar was killed in a heinous act of violence … while attempting to stop two vehicles that illegally entered the country and were absconding into Mexico,” Homeland Security Secretary Michael Chertoff said June 25 in denouncing Mr. Navarro’s release. “We are working with a determined Mexican government, and our Department of Justice, to seek swift justice for the Aguilar murder.”

In January, the Mexican Embassy, which announced Mr. Navarro’s arrest, said a preliminary investigation found that he left Mexicali and was headed for the U.S. “driving a Hummer vehicle, presumably carrying drugs.” The embassy said that as Border Patrol agents tried to stop the vehicle, “Agent Aguilar was run down, and Mr. Navarro fled the scene back to Mexican territory.”

On Monday, Mr. Alday said the initial information the Mexican government had was based on what it was told, not on any evidence that was provided. He said Mexican authorities are “working cooperatively” with the U.S. in a hunt for Mr. Navarro, whose whereabouts is unknown.

“Once he’s located, the arrest warrant based upon the provisional request for extradition purposes will be executed,” he said.

Agent Aguilar, 32, was a six-year Border Patrol veteran assigned at the agency’s Yuma sector office. He was married and the father of two children.