The Air Force said it failed to follow procedures and alert federal law enforcement about Devin Kelley's violent past, which allowed him to obtain firearms. (Video: Elyse Samuels/The Washington Post, Photo: Carolyn Van Houten/The Washington Post)

Devin Patrick Kelley's co-workers didn't think much of it when he failed to show up Sunday for his evening shift as a security guard at Summit Vacation and RV Resort.

Kelley had been 35 miles north in Sutherland Springs, police say, committing one of the deadliest mass shootings in modern American history. But to his co-workers, he was just a new worker on the job. In his five weeks there, nothing about him set off alarms.

“It’s scary,” said Claudia Varjabedian, the park’s office manager, who said Kelley passed a background check before he was hired for the full-time job. Kelley was responsible for after-hours check-ins and making sure the pools, cafe and clubhouse were locked at 10 p.m.

“You don’t know who you work with sometimes,” Varjabedian said.

Police say that Kelley, 26, was the gunman whose shooting rampage at a small Southern Baptist Church on Sunday left 26 dead and more than 20 wounded. After fleeing the scene, he was confronted by at least one armed resident and took his own life soon after, police said.

Law enforcement identified 26-year-old Devin Kelley as the gunman who killed dozens at the First Baptist Church in Sutherland Springs, Tex., on Nov. 5. (Video: Amber Ferguson, Elyse Samuels/The Washington Post)

Police and court records in three states paint the picture of a young former U.S. airman with a sometimes violent private life. In briefings Monday, local officials — who emphasized that the investigation is ongoing — said that at least one of Kelley’s relatives attended First Baptist Church in Sutherland Springs, and that the shooting was proceeded by a “domestic situation.”

Kelley graduated from New Braunfels High School in 2009, a spokeswoman for the school district said. His senior photo was the only image of him in the yearbook.

“We are shocked to hear that a graduate of our lone high school is allegedly responsible for this tragedy,” read a statement from spokeswoman Rebecca Villarreal. “This senseless act of violence is something that is hard to understand and has definitely shaken our community. We grieve with those that suffered a loss and offer our deepest condolences.”

The lives lost in Sutherland Springs, Tex.

In 2010, Kelley enlisted in the Air Force, serving as a logistical readiness airman at Holloman Air Force Base in New Mexico, according to Air Force spokeswoman Ann Stefanek.

Kelley’s string of legal problems began as early as 2012, when he was court-martialed in New Mexico and sentenced to a year in military prison for assaulting his then-wife and her child, making him part of a long line of mass attackers or suspects with domestic violence in their pasts.

According to court-martial documents released Monday evening by the Air Force, Kelley was found guilty of domestic violence. Prosecutors alleged that on June 21, 2011, and April 27, 2012, he unlawfully struck, choked, kicked and pulled the hair of his wife and struck her young child “with a force likely to produce death or grievous bodily harm.”

Court records in nearby Alamogordo, N.M., show that in May 2012, Tessa K. Kelley filed for divorce from her husband, who she said was in jail at the time.

After his prison sentence, Kelley was reduced in rank and released from the military with a bad-conduct discharge in 2014.

The military sentence raises key questions for investigators about how Kelley obtained his weapons. Officials said they recovered at least four guns from Kelley’s vehicle, but also said that Kelley had sought and failed to obtain a permit allowing him to carry a concealed weapon. Officials with the Air Force said Monday that his conviction — which should have prevented him from purchasing weapons — had not been properly flagged for the FBI and that they would launch an internal investigation.

The Air Force says it failed to follow procedures, allowing Texas church shooter to obtain firearms

In August 2014, Kelley was charged with a misdemeanor count of mistreatment, neglect or cruelty to animals in nearby El Paso County, Colo., where he lived at one point, records show. Sheriff’s deputies responded to a call about a man who was punching a dog, police records indicate. Four witnesses told deputies that they saw a man matching Kelley’s description yelling at and chasing a white-and-brown husky.

“The suspect then started beating on the dog with both fists, punching it in the head and chest,” a deputy wrote in the incident report. “He could hear the suspect yelling at the dog and while he was striking it, the dog was yelping and whining. The suspect then picked up the dog by the neck into the air and threw it onto the ground and then drug him away to lot 60.”

Kelley was charged with animal cruelty and the dog was transferred to the Humane Society for a full medical evaluation.

This summer, Kelley worked briefly as an unarmed night security guard at a Schlitterbahn water park in New Braunfels, the company said. He passed a Texas Department of Public Safety criminal background check before beginning work there, a spokeswoman said, though she added that Kelley was fired in July — as the season was reaching its peak — because he was “not a good fit.”

He was also able to pass a background check that allowed him to work for HEB, a Texas grocery chain, in New Braunfels. Company spokeswoman Dya Campos said he worked there for two months in 2013 and quit; she was unsure of his position there.

Records indicate that Kelley lived for some period on a property valued at about $800,000 owned by his parents in New Braunfels. The secluded home sits on 28 acres of wooded farmland, separated from the nearest main road by a long private driveway.

Neighbors told local news media that Kelley lived in a barn behind the 3,700-square-foot home with his current wife and 2-year-old son. They said the family had lived there for more than a decade.

Dave Ivey, who identified himself as Kelley’s uncle, apologized to the shooting victims in an interview with NBC News.

“I never in a million years could have believed Devin could be capable of this kind of thing,” Ivey said. “My family will suffer because of his coward actions.”

Cars lined the highway outside the house on Monday morning. A Comal County sheriff’s truck blocked the property’s gate, which had a “Beware of dog” sign.

Doug, who lives across the street and declined to give his last name, said he didn’t get to know the family at all, in the 11 years he lived on the same road.

“The only time I see them is when they’re going out and they don’t even look my way,” he said. He said he didn’t recognize the shooter.

He said he regularly heard gunshots coming from the property across the street but thought little of it. The noise used to rattle his two small dogs, he said.

Mark Moravitz, who lives across the street from the Kelley family, also said he frequently heard gunfire coming from the property, often at 10 or 11 p.m.

“We hear a lot of gunfire,” he told KSAT, “but we’re out in the country.”

Moravitz told local media that the Kelley family traveled frequently, so he would housesit for them. He described Kelley as a “regular guy” and said it was “shocking” to hear about the shooting. “You never think your neighbor is capable of something like that,” he said. “If he did that, that kind of worries you, thinking we’ve been living next door to the guy.”

A Facebook page bearing Kelley’s name showed a photo of a Ruger assault-style rifle — the kind of weapon police say was used in the shooting. The page was taken down on Sunday.

Former high school classmates of Kelley took to their own Facebook pages in shock, describing him as a social outcast whom some had blocked or deleted from their social networking because he sent inappropriate or aggressive messages.

“I was close with Devin Kelley from middle school through high school . . . and I had always known there was something off about him,” Courtney Kleiber wrote on Facebook.

“He use to be happy at one point, normal, your average kid,” she wrote. “Over the years we all saw him change into something that he wasn’t. To be completely honest, I’m really not surprised this happened, and I don’t think anyone who knew him is very surprised either.”

About a 15-minute drive from his family’s ranch, Kelley’s co-workers at the RV resort said they knew little about him. On Monday, the grounds appeared nearly empty except for a few maintenance workers driving golf carts and clearing out trash. Co-workers said Kelley stood out only because of how quiet he was, which at times drew comments from park guests.

It is now a frustratingly familiar trope: the mysterious inner life of a quiet stranger at the workplace.

“He didn’t seem like the kind of person that would do something like that,” Varjabedian said, adding that Kelley had done a “fairly good job” while he worked at the RV park.

“We don’t know a thing,” she added. “That’s the problem.”

Joel Achenbach in New Braunfels, Bob Moore in Alamogordo, N.M., and Sandhya Somashekhar, Alex Horton, Derek Hawkins, Julie Tate, Scott Wilson and Travis Andrews in Washington contributed to this report.