WINTHROP — Investigators are scouring the background and personal writings of the gunman who killed two Black people in this coastal suburb to determine what precipitated the attack and fueled his extremist views, authorities said Monday.
Nathan Allen, 28, who authorities say left behind writings that described white people as “apex predators,” acted with “hate in his heart” when he gunned down retired state trooper David Green, 68, and Air Force veteran Ramona Cooper, 60, before he was killed in an exchange of gunfire with a Winthrop police sergeant, Suffolk District Attorney Rachael Rollins said at a news conference.
“When he became radicalized, or when he started believing these things, we are not certain yet,” Rollins said. “We are continuing to, every minute, to investigate this, to interview people, and to find out why and how this happened.”
On Saturday afternoon, Allen crashed a stolen truck into a building on Shirley Street and then “executed” two bystanders a short distance away, authorities said. He shot Cooper in the back multiple times and shot Green repeatedly in the head, neck, and torso. Allen passed several other people after the crash who were not Black but did not harm them, Rollins said.
“We are learning more every day, but I am confident saying that there was hate in this man’s heart,” she said. “Whether that was the only reason he did what he did, we’re not going to be fully sure. But I can tell you I’m certain of his language, in his own handwriting, and certain of the fact that the two people that were murdered are Black.”
Kim Carrillo, a neighbor, said Allen was carrying two guns when he charged at the officer.
“He went right past us,” she said Monday. “He didn’t touch us. He looked right at us.”
Allen “just went toe-to-toe with the cop,” said Carrillo, who is white. “He could’ve killed us.”
The officer shot Allen when his weapon jammed, Carrillo said.
Authorities are investigating the shootings as hate crimes. They believe Allen, who lived in Winthrop and had no criminal history, acted alone, Rollins said.
Rollins said she had gained insight into Allen’s mindset from writings he left behind, including notebook entries from within the past week. His writings included white supremacy rhetoric and drawings of swastikas.
Allen held “some very disturbing beliefs, white supremacist beliefs, regarding members of our Jewish population, as well as Black individuals,” she said. “And two people lost their lives as a result of it.”
Rollins said her office is working with the FBI and state and local agencies as part of the investigation.
FBI spokesman Kristen Setera said Allen was not known to the agency. “The FBI has not received any reporting on Nathan Allen, nor have we had any interactions with him,” she said in a statement.
Over the weekend, police evacuated the building where Allen lived so they could investigate packages for him in the lobby, neighbors said.
Allen, who had a doctorate and was married, likely appeared unassuming, Rollins said. Authorities said they do not know where Allen was driving when he crashed, traveling twice the speed limit, but noted Jewish houses of worship were nearby.
“There is a growing national, and global, problem with extremism and white supremacy. The FBI believes the most serious domestic violent extremist threat comes from ‘racially or ethnically motivated violent extremists, specifically those who advocated for the superiority of the white race,’ ” Rollins said over the weekend.
Officials said they were launching a multifaceted effort to help residents cope with the aftermath of the violence. The efforts will include going door-to-door to check if people need support, a neighborhood drop-in meeting on Wednesday, and a vigil for the two victims at Town Hall on Thursday.
On Monday, grief-stricken relatives and neighbors remembered the two victims fondly. Green’s brother, Aria Green, said he believed his brother was trying to jump the shooter when he was killed.
“I believe David could have left the scene if he wanted to. He had the opportunity, but I think he was trying to stop this guy from doing any more damage, shooting any more people,” said Green, 70. “He was a hero to me. He was just that type of person. I think the word ‘hero’ has been kind of overused and used for just about anything. You play sports, you’re a hero. But Dave was a hero.“
Green said his brother leaves a proud legacy.
“I just want people to remember that my brother was not only a brave person but he was a very kind and good person,” he said. “He wanted to help people. I think everyone’s going to miss him.”
Neighbors described how a sleepy summer afternoon exploded into chaos. Bill Leach, a neighbor of David Green’s, said they were standing together outside when they heard a loud crash. When they walked down the street to find out what happened, they saw Allen gun down Cooper.
At the command of a police officer, Leach and another neighbor took cover. When he looked up, Allen was “standing right beside us.”
“I looked at him and I just shut my eyes because I didn’t want to see it coming,” he said. “I’m thinking he’s shooting everybody.”
Leach heard an officer yell at Allen to drop his weapon and then saw an officer shoot him. When Leach went to tell Green, he found him lying on the ground and yelled for help.
Kim Carrillo said she was getting dressed to get a slushy down the street when she heard the crash. She poked her head outside her door, which opens onto Shirley Street, and then ran to help Cooper, who was shot on the next block.
Carrillo said she ran past the alleyway where Green was killed.
“I ran down to help the lady because I wanted to help the lady. That’s how I am. When I was running, Dave was right here on the ground,” Carrillo said. “I didn’t see Dave until I came back. Dave was already gone.”
Louise Corkhum, 78, said she knew Green as a gentleman who was exceedingly kind.
“He was the first one always to help me carry in my bundles,” she said. “It’s so sad.”
Cooper had participated in a “compensated work therapy” program and had worked at the store in the Veterans Administration facility in Jamaica Plain, said Kyle Toto, a spokesman for the VA’s Boston office.
“She was a beloved colleague who will be greatly missed,” Toto said by e-mail.
According to the VA website, the CWT program provides ”employment opportunities for veterans with physical and mental health challenges.” It was not clear when Cooper last participated in the program.
Cooper’s former husband, Gary Cooper Sr., said Monday that Cooper had most recently been living in Chelsea. He said the family wasn’t certain why she was in Winthrop Saturday.
They were married from 1983 to 1995 while they served in the Air Force together, he said. Their son, Gary Cooper Jr., 32, was born while they were stationed in Germany.
When the couple traveled “she liked to go to the ocean,” Gary Cooper Sr. said. She grew up in West Haven, Conn., and “could look out and see the ocean from her homeroom class,” he said.
She and her identical twin sister, Regina, were adopted at birth, he said. Her brother and parents have died.
”She didn’t have much family left,” he said.
Martin Finucane, Travis Andersen, and John R. Ellement of the Globe staff contributed to this report.