Mexican military officials said the ringleader was Julian Zapata Espinosa, who is reputed to be a cell leader of the Zetas crime and drug syndicate. He told investigators that the shooting was caused by "confusion" about the vehicle he had been chasing - indicating that U.S. agents may not have been the intended target.

Five more men were arrested in the case, all alleged to be gang members. They hobbled with Espinosa onto the stage at a news conference Wednesday night, some sporting swollen faces and busted lips.

Jaime Zapata, a special agent for U.S. Immigration and Customs Enforcement, was killed by gunmen Feb. 15 as he and fellow agent Victor Avila were traveling in an armored sport-utility vehicle with diplomatic plates four hours north of Mexico City.

Avila was wounded in the ambush.

ICE director John Morton called the arrest "a welcome development" but cautioned the investigation was ongoing.

The arrest might help improve strained relations between the United States and Mexico. The White House announced Wednesday that Calderon would pay a visit to President Obama next Thursday.

In a sensational interview with El Universal newspaper Tuesday, Calderon vented his anger at leaked diplomatic cables that describe Mexico's anti-crime efforts as hobbled by corruption, human rights abuses and lack of cooperation. The cables, released by the anti-secrecy Web site WikiLeaks, also suggest that most of the big arrests in the anti-drug fight come about with U.S. intelligence spoon-fed to the Mexicans.

Calderon lashed out at U.S. Ambassador Carlos Pascual, saying his "ignorance has translated into a distortion of what is happening in Mexico." Calderon also expressed anger at repeated accusations by U.S. officials that Mexico is a "failed state" facing "an insurgency" that could require U.S. intervention along the border.