Border Patrol agent shot dead in Arizona, another wounded PHOENIX (Reuters) - A Border Patrol agent was shot dead and another wounded when they came under fire early on Tuesday while responding to a tripped ground sensor in a drug smuggling corridor in Arizona, near the border with Mexico, authorities said. Authorities said three agents were on foot about 5 miles north of the border when gunfire erupted well before daybreak but provided few additional details on the circumstances of the violence. "As they were walking up the trail, they reported taking gunfire," Cochise County Sheriff's spokeswoman Carol Capas said. We have unknown suspect or suspects at this point. The shooting marks the fourth death of a Border Patrol agent in Arizona in less than two years and was likely to reignite concerns over border security in a state neighboring Mexico that is already at the forefront of the national immigration debate. Flags will be lowered in honor of the slain agent. Elected officials will vow to find those responsible. Arizonans and Americans will grieve, and they should. But this ought not only be a day of tears," Governor Jan Brewer said in a statement. There should be anger, too. Righteous anger - at the kind of evil that causes sorrow this deep, and at the federal failure and political stalemate that has left our border unsecured and our Border Patrol in harm's way," added Brewer, a vocal foe of President Barack Obama's administration on immigration. Brewer, citing what she described as a federal failure to secure Arizona's southern border, signed a broad immigration crackdown into law in 2010 to try to crack down on the flow of illegal immigrants into the state where an estimated 360,000 undocumented people live. Critics of the law, which include a requirement that police check the immigration status of anyone they stop and suspect of being in the country illegally, have said it could lead to racial profiling. The shooting took place near the border town of Naco, southeast of Tucson, which remains a corridor for marijuana trafficking and human smuggling, despite the construction of a tall, steel fence along the border. "We need to redouble our efforts to secure the border and ensure the safety of Border Patrol agents," U.S. Democratic Representative Ron Barber, who represents the southern Arizona district where the shooting occurred, said in a statement. Sheriff's deputies were called to the scene at 1:33 a.m. local time (4:33 a.m. EDT/0833 GMT) and found one agent dead and another with non-life-threatening injuries, Capas said. A third was unharmed. FBI agents were also investigating. The Border Patrol identified the slain agent as Nicholas Ivie, 30, who was originally from Utah and had worked for the agency since 2008. The agents had been responding to a sensor, which picks up on movement or vibrations in areas authorities suspect are used by drug traffickers and illegal immigrants. When an alert is triggered, agents have the option to respond. Capas said the agents who were shot were assigned to the Brian A. Terry Border Patrol Station, named after an agent whose 2010 death in the line of duty in Arizona borderlands was linked to a botched U.S. operation to track guns smuggled to Mexico. In that case, two guns tracked by the Bureau of Alcohol, Tobacco, Firearms and Explosives in the "Fast and Furious" sting operation, which let weapons slip into Mexico, were retrieved from the spot where Terry died in a shoot-out with bandits. It was unclear if the weapons were used in his murder. Separately, two Border Patrol agents were killed last year in a accident during a car chase with smugglers near Gila Bend, near Phoenix. Reporting by David Schwartz in Phoenix, Tim Gaynor in Mexico City, and Daniel Trotta in New York; Writing by Cynthia Johnston; Editing by Stacey Joyce and Cynthia Osterman