A U.S. Border Patrol agent in Rio Bravo, Texas, shot and killed an undocumented immigrant Wednesday, according to US Customs and Border Protection (CBP).CBP said in a news release the law enforcement officer was responding to a report of "illegal activity near a culvert where he discovered the group of three migrants."During the encounter, the agent shot the migrant in the head."Initial reports indicate that as the agent attempted to apprehend the group, he came under attack by multiple subjects using blunt objects," the statement said.A Webb County Medical Examiner said officials are in touch with the Guatemalan consulate to identify the woman.The names of the agent and the deceased have not been released.A local woman, Marta Martinez, who lives on Centeno Lane where the incident happened, livestreamed the aftermath on Facebook.Martinez said she heard the shot around 12:20 p.m. and decided to go outside to see what was happening.In the video, Martinez said she saw a woman lying on the ground and border patrol agents walking out of what looked like an abandoned building with two people in handcuffs."Why are you mistreating them? Why are you mistreating them? Why did you [shoot] at the girl? You killed her. He killed the girl. She's laying there and she's dead," Martinez said in the video.Three people were detained by agents.Martinez told BuzzFeed News she saw agents "flip the woman over" and "she was bleeding heavily from the left side of her face."The officer, who shot the woman, gave her CPR until emergency responders arrived at the scene, the statement said."The agent fired at least one round from his service issued firearm, fatally wounding one of the assailants. The rest of the group fled the scene," according to CBP.Fire Chief Juan C. Gonzalez of Rio Bravo Fire & Rescue told CNN Gonzalez still had a pulse when they arrived around 12:47 p.m. However, by the time rescue team got supplies, Gonzalez no longer had a pulse.Rio Bravo is on the border with Mexico, about 170 miles south of San Antonio.The incident is under investigation by the FBI and Texas Rangers.FBI agents are expected to interview the three people taken into custody, the law enforcement source told CNN on Thursday.Martinez said federal agents are often seen patrolling the street since it comes to a dead end. Because of its proximity to the Rio Grande River, it is a popular place for people who want to cross the border illegally.Martinez told the New York Times she often sees officers "chasing after suspected undocumented immigrants," but in the 20 years she has lived there, she said she had never heard of a Border Patrol agent fatally shooting someone.