NORFOLK, Va. (AP) — A city engineer who fatally shot 12 people in a Virginia Beach municipal building in 2019 “was motivated by perceived workplace grievances” that “he fixated on for years,” according to findings released by the FBI on Wednesday.

The investigation, conducted by the FBI’s Behavioral Analysis Unit, found that DeWayne Craddock “struggled with how he perceived his own work performance and how others at work viewed him.”

“The shooter’s inflated sense of self-importance contributed to this conflict and led him to believe he was unjustly and repeatedly criticized and slighted,” the FBI said in a news release. “Violence was viewed by the shooter as a way to reconcile this conflict and restore his perverted view of justice.”

But the FBI cautioned that no person or group was in a position to “see the confluence of behaviors that may have forewarned the attack” because Craddock had purposely isolated himself and disengaged from his relationships.

The agency also said that Craddock suffered from significant mental health stressors, although they “alone cannot explain the Virginia Beach attack.”

“It is important to note that only the shooter knew the real reason why he committed this horrific act of violence; however, at this time, the FBI is confident, based on evidence collected, that the above assessment is accurate,” the FBI stated.

The FBI’s findings appear to go a step further than two previous investigations into the mass shooting in the coastal city of nearly half a million people.

Virginia Beach police said in March that they could not determine a motive.

“Despite exhaustive investigative work and in spite of unsubstantiated rumors and accusations, it appears we may never know why he committed this heinous act,” the city’s report concluded.

Craddock had worked in the city’s public utilities department for more than nine years. He killed 11 co-workers and a contractor who was in the building at the time getting a permit. Four others were seriously wounded and a police officer responding to the shooting received a bullet in his tactical vest but escaped serious injury. Craddock was killed in a shootout with police.

The city’s report had said Craddock’s life began to change around 2017. He was getting a divorce and started to have performance issues at work. In 2018, he received a written reprimand for poor performance, failed to meet expectations on an evaluation and didn’t get a merit raise.

Leaders in the department said the shooter would have met job performance standards in his 2019 evaluation, the report said.

The city’s report said investigators didn’t uncover “any indications of violent tendencies or acts of violence committed by the suspect prior to May 31, 2019.”

It echoed similar findings released by Chicago-based security company Hillard Heintze in November 2019. The firm offered no clear motive and said that Craddock “did not work in a systemically toxic workplace.”

The FBI’s report, however, is somewhat validating for some of the victims’ families who have long blamed what they say was a toxic workplace environment and a failure by supervisors to recognize warning signs.

Jason Nixon, whose wife, Kate Nixon, was killed, has long said that the shooter was upset because he was having trouble at work and lost out on a promotion.

“Human resources dropped the ball on policies, protocol and procedures,” Nixon said in March. “My wife warned them all the time that there’s something wrong with this guy.”

Nixon said Wednesday that he expects little to come of the FBI’s report because Virginia Beach “plays by its own rules.”

Hillard Heintze did offer several recommendations in its 2019 report to Virginia Beach, such as improving its workplace violence prevention programs and human resources functions. City spokeswoman Julie Hill wrote in an email Wednesday that Virginia Beach has implemented — or is in the process of implementing — a number of those recommendations.

Mayor Robert M. “Bobby” Dyer said in a statement Wednesday that “no matter how much information we have about some situations, it may never be enough to really explain what happened and why … we have seen the worst evil one person can do, and the very best of what a community can do when people come together.”

The rampage in Virginia Beach had been the latest in a string of high-profile mass shootings, happening in between the high school killings in Parkland, Florida, that left 17 people dead and the Walmart massacre in El Paso, Texas, that left 23 dead.

Some of the victims’ family members have felt that the tragedy was effectively forgotten after the national spotlight moved on to other mass killings. Then the coronavirus pandemic arrived.

The Virginia Beach mass shooting is still under investigation by a state commission that was formed to look into the tragedy. It held its first meeting Wednesday morning, according to the Kate Hourin, a spokeswoman for the Virginia Office of the State Inspector General.

The state IG’s office is providing staff support for the commission, which will investigate Craddock’s possible motives and develop recommendations to minimize the risk of similar shootings.