A federal immigration agent told his former wife that he was the person who fatally shot a Colombian immigrant in Maine this week and defended his actions to her, the ex-wife said.

The woman, Ashley Brouillette, identified the agent as David Brouillette, 37, in an interview on Thursday. She said that when she spoke to him on Wednesday about the shooting, Mr. Brouillette said that it was justified.

He told her not to talk to anyone about him, she said, but then asked her to defend his character. “He said, ‘You need to tell them I’m a good person,’” she recalled Mr. Brouillette saying, adding that he asked her not to tell anyone that he had abused her during their marriage. “I told him that I wouldn’t lie for him,” Ms. Brouillette, 37, said.

The Department of Homeland Security has not released the name of the shooter, and a spokeswoman for the agency declined to confirm whether Mr. Brouillette had fired the fatal shots, saying that publicizing agents’ names puts them at risk.

Ms. Brouillette said she had spoken to her ex-husband about the shooting and had also seen video and images of him at the scene. Two other people — one a relative who asked not to be identified and another a high school friend of Mr. Brouillette’s — said they also recognized him from video footage and witness photos, but could not confirm that he was the person who fired the fatal shots.

No video shedding light on who fired the fatal shot on Monday has emerged. But one video recorded just after the shooting appears to show another agent consoling Mr. Brouillette.

Mr. Brouillette was first reported as the agent who fired the shots by The Portland Press Herald.

The man who was killed, Johan Sebastián Durán Guerrero, 25, lived with his partner and 3-year-old daughter in Biddeford, Maine. He had been driving away from his home early on Monday morning when agents with Immigration and Customs Enforcement confronted him. Multiple gunshots rang out, leaving holes in the windshield of Mr. Guerrero’s car.

Later in the week, the Department of Homeland Security said in a statement that Mr. Guerrero illegally entered the United States on Sept. 1, 2023, via the southern border. Benjamin Gideon, a lawyer representing the Guerrero family, said on Thursday that Mr. Guerrero had been working in the United States lawfully.

A phone number listed in court records as Mr. Brouillette’s has been disconnected, and a message left at another got no response. He did not respond to an email seeking comment.

According to the Department of Homeland Security, the ICE agents in Biddeford had been monitoring what they believed to be the residence of someone who was in the country illegally, for whom they had a removal order.

In a statement Monday, the department said an officer had opened fire, “fearing for public safety.”

When Ms. Brouillette spoke to him on Wednesday about the shooting, she said, Mr. Brouillette defended it as justified. “I asked him why he did it, and he said, ‘The guy tried to hit me,’” she said.

The statement did not name the person the agents had been seeking, but in the hours after Mr. Guerrero was killed, details emerged to suggest that he was not the target of their search.

Ms. Brouillette, who dated Mr. Brouillette throughout high school and was married to him from 2007 to 2009, said her former husband had become abusive after he joined the military in 2007. She said he had choked her, slammed her to the floor and pushed her against a wall.

Mr. Brouillette’s threats and harassment were so persistent that she left Maine in 2012 to escape him, she said.

Still, she said, he had threatened to kill her as recently as November after she disciplined their daughter by taking away her cellphone. “I have said to people, ‘If I end up dead, it’s him,’” Ms. Brouillette said.

Another woman identified as a former wife of Mr. Brouillette’s in court documents has also accused Mr. Brouillette of abuse. In a filing for an order of protection in December 2019, obtained by The New York Times, the woman wrote that he had broken her door down, destroyed her belongings and dumped her clothes over a bridge. The filing also said that Mr. Brouillette had spit in her face and cornered her in a room.

“I am scared of him coming to my home or sitting in the parking lot of my work until I leave,” she said in the filing.

The woman could not be reached for comment.

Ms. Brouillette said her former husband had post-traumatic stress disorder related to his military service in Afghanistan in 2012 and 2013; in court records, the other woman identified as a former wife also said he suffered from PTSD.

According to Army Public Affairs, he served from 2007 to 2015. He enlisted as a quartermaster and chemical equipment repairer and then served as a medical logistics specialist. He also served in the Maine Army National Guard from 2007 to 2010. From 2010 to 2015, he served as a human intelligence collector in the Army. He left the Army as a sergeant.

He also worked as a police officer for the Department of Veterans Affairs in Maine, Ms. Brouillette said.

Ms. Brouillette said that her former husband should not have been hired to work in immigration enforcement. It was not clear when he joined ICE. “He has mental health issues, he’s short-tempered, he’s reactive,” she said. “He should be in treatment.”

Scott Collins, 37, said he was best friends with Mr. Brouillette throughout high school in Gardiner, Maine, outside Augusta, the state capital. In text messages to The Times on Friday, Mr. Collins described Mr. Brouillette as violent and “prone to start fights” and said he had long aspired to join law enforcement.

After completing high school, Mr. Brouillette lived with Mr. Collins’s grandparents because his mother had kicked him out of her house, Mr. Collins said.

Mr. Collins, a line cook who lives in Augusta, said he last saw Mr. Brouillette in 2016, when the two tried to reconnect over drinks at an Applebee’s. “He honestly reminded me of the same person I knew in high school,” Mr. Collins said. “It was almost like he never changed.”

The fatal Maine shooting, which followed another in Houston during a traffic stop by federal immigration agents, prompted vigils and protests in Biddeford, as well as in the nearby cities like Portland and Scarborough, the site of an ICE facility.

The Trump administration has recruited thousands of new ICE agents over the past year to carry out a nationwide immigration enforcement surge, and it appears to have cut training requirements.

At least 23 people have been shot at by federal immigration agents around the country since last year. Six have died, including three U.S. citizens.

Mark Arsenault and Murray Carpenter contributed reporting. Kitty Bennett, Georgia Gee and Sheelagh McNeill contributed research.