A former Border Patrol agent broke down in tears as he described desperately trying to save the life of a colleague who was shot during a firefight that exposed the bungled federal gun operation known as Fast and Furious.

William Castano was among the first witnesses Wednesday in the trial of two suspects charged in the 2010 death of Brian Terry. It is the first criminal trial in Terry's killing, which brought to light the government's operation that allowed criminals to buy weapons with the intention of tracking them. Instead, they lost about 1,400 guns, including two found where Terry was killed.

Castano became emotional in federal court in Tucson as he walked the jury through the night of Dec. 14, 2010, when he, Terry and two other agents were on a mission to arrest gangs known as rip crews that target marijuana smugglers.

The agents yelled "policia" after spotting the weapon-toting crew, Castano said. They later fired beanbag rounds from shotguns at the smugglers, and a gunfight ensued. Terry was hit in the back. Castano cut open Terry's shirt to try to locate the wound, using a flashlight to guide him.

"I put my hand all over his body to see if I could feel blood," he said.

Terry lost consciousness. Other agents arrived to help carry him down a hill, which was no easy task given his muscular, 215-pound frame, Castano said. Castano lost his composure while testifying.

Opening statements began earlier in the day in the trial of Jesus Leonel Sanchez-Meza, also known as Lionel Portillo-Meza, and Ivan Soto-Barraza. Two others have already pleaded guilty in the case, and another two remain fugitives.

Defense attorney Ramiro Flores was quick to point out that the agents deployed their beanbag shotguns first and three of the men ran away. "Someone triggered that firefight, and it wasn't these individuals here," Flores said of the defendants.

Flores said he would touch on the Border Patrol's use-of-force policy during the trial. The agency has come under heavy criticism over allegations that agents too often use deadly force against immigrants, often in response to those who throw rocks.

Prosecutor Todd Wallace Robinson said DNA pulled from water bottles and sweaters left behind by the alleged rip crew matched Sanchez-Meza and Soto-Barraza, and that the men confessed after being found in Mexico several years later that they were part of the crew.

"All five members of the rip crew were carrying weapons, and they were carrying them for one purpose and one purpose alone, and it was to rob smugglers," Robinson said.

Sanchez-Meza and Soto-Barraza face charges of first- and second-degree murder, assault on a federal officer, conspiracy to commit robbery, attempted interference with commerce by robbery and carrying a firearm during a crime of violence.

Terry's death brought to light the Fast and Furious operation, which quickly became a hot political issue in Washington. Republicans sought to hold the Obama administration accountable over the operation, conducting a series of inquiries into the how the Justice Department allowed guns to end up in the hands of criminals.

Former Attorney General Eric Holder was held in contempt after he refused to divulge documents for a congressional investigation into the matter. Since then, the Justice Department has focused on arresting and trying all suspects involved.

Information about the operation will be excluded from testimony in trial, U.S. District Judge David C. Bury ruled recently.

Manuel Osorio-Arellanes, one of the men present but likely not the shooter, pleaded guilty to first-degree murder and was sentenced in February 2014 to 30 years in prison.

Rosario Rafael Burboa-Alvarez, accused of assembling the armed crew, struck a plea deal last month that will likely result in a 30-year prison sentence, with credit for time served. He will be sentenced in October.

Two other suspects are still on the loose.