PHOENIX -- Investigators searching a stretch of the U.S.-Mexico border for clues into the fatal shooting of a Border Patrol agent face a treacherous territory that is heavily used by drug smugglers, offers many hiding places and is close enough to Mexico for traffickers to make a quick getaway.

Agent Nicholas Ivie was killed and another agent wounded in the sparsely populated desert in southeastern Arizona on Tuesday.

Those who carried out the shooting near Bisbee, Ariz., probably had time to cross the border in the early-morning darkness before authorities could seal off an escape route, said George McCubbin, president of the National Border Patrol Council, a union representing about 17,000 border patrol agents.

"I seriously doubt anybody would be laid up and hiding," he said.

Ivie and two other agents were fired upon in a rugged hilly area about five miles north of the border as they responded to an alarm that was triggered on one of the sensors the government has installed along the border.

The wounded agent was shot in the ankle and buttocks and released from the hospital after undergoing surgery. A third agent was not injured.

Ivie, a 30-year-old father of two, grew up in Utah and was active in the Mormon church. He had been an agent for four years.

Authorities have declined to provide other details, including what they believe prompted the shooting and whether the agents were ambushed. Still, they suspect that more than one person fired on the agents. No arrests have been made.

The last Border Patrol agent fatally shot on duty was Brian Terry, who died in a shootout near the border in December 2010.

Terry's shooting has been linked to the government's "Fast and Furious" gun-smuggling operation, which allowed people suspected of illegally buying guns for others to walk away from gun shops with weapons, rather than be arrested.

Authorities intended to track the guns into Mexico. Two rifles found at the scene of Terry's shooting were bought by a member of the gun-smuggling ring being investigated.

Critics of the operation say any shooting along the border now raises the specter that those illegal weapons are still being used in border violence.

A federal law enforcement official spoke on the condition of anonymity and said no weapons have been found. The official requested anonymity because information on the search has not been released publicly.

The latest shooting occurred in an area heavily frequented by drug smugglers, though less so in recent years by illegal immigrants crossing the border, said Dave Stoddard, a retired Border Patrol agent who worked in the agency's station in the area for eight years. "You're talking about cocaine alley," he said.

"It takes maybe less than a minute and everybody leaves the scene," Stoddard said, explaining that the "mules" either get in the vehicle or go back into the desert to return to Mexico on foot.

The desert where the shooting occurred is part of the nation's busiest Border Patrol sector, which received additional agents and fencing as the federal government sought to improve border security in recent years.