A U.S. Border Patrol agent is dead, shot in a gun battle Tuesday night at the U.S.-Mexico border near Rio Rico, Arizona.

Agent Brian Terry, 40, was killed when he and other agents exchanged fire with five men they came upon in Peck Canyon, according to FBI spokesperson Brenda Nath.

"The agents were fired upon by the suspects. They returned fire. In the exchange Agent Terry, unfortunately, was killed," Nath said.

The incident happened about 11:15 p.m. Tuesday, Nath said.

Four of the suspects were apprehended and are now in custody. A fifth suspect is at large, authorities said. One of the four in custody was injured in the fight and is in a hospital in Tucson, the FBI said.

Published reports say that the five suspects were at that place, which is close to the border, to prey upon and rob illegal aliens crossing from Mexico.

Department of Homeland Security Secretary Janet Napolitano called the killing "an unconscionable act of violence."

"It is a stark reminder of the very real dangers our men and women on the frontlines confront every day as they protect our communities and the American people," Napolitano said.

Napolitano said that authorities would spare nothing until everyone involved in Agent Terry's death was brought to justice.

"Our thoughts and prayers are with the Terry family for their tragic loss," said Customs and Border Patrol Commissioner Alan Bersin. "Our commitment to Agent Terry and his family is that we will do everything possible to bring to justice those responsible for this despicable act."

The investigation is being led by the FBI and the Santa Cruz County Sheriff's Office.

Terry is the first Border Patrol agent killed in the line of duty since July 2009.

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