The gunman who opened fire inside a luxury condominium building in Washington, D.C., on Thursday morning had been complaining about mold and water problems in his apartment before the deadly attack, which police said targeted the building's manager.

One person was killed and four others were wounded in the shooting in the Logan Circle neighborhood. Police identified the slain victim as 46-year-old James Stuart, who managed the building. The gunman, identified as Dr. Matt Lambert, 55, shot and killed himself in his apartment after a standoff, police said. Three firearms were recovered from the unit, according to law enforcement sources in Washington.

An account on X under the name @OfMiceandMold served as a running archive of complaints about the alleged disrepair of the apartment where Lambert lived. In dozens of posts, he vented frustration with the Metropole Condominiums' building contractors, EJF Real Estate Services, alleging they failed to address problems with his unit.

Lambert, who ran a D.C.-based healthcare consultancy, purchased the condo in September 2016 for $1,095,000, according to D.C. tax records, and court documents show he owned a penthouse unit in the building. Before the shooting, he posted what he described as an engineering report that investigated mold and water issues. "My manifesto is an engineering report," he wrote.

In an Oct. 1 post, he appeared to invoke Luigi Mangione, who shot and killed UnitedHealthcare CEO Brian Thompson in 2024. "After months of 'Luigi' tactics (delay, deny, defend) by the HOA, we were finally able to submit an independent engineering report into the record," he wrote, adding that he would share the report "when the time is right." Mangione had written "delay, deny, defend" on the bullets he used in the shooting. In an April 28 post, Lambert wrote: "If you're looking for the building manager, I would check the gym."

Lambert had sued the Metropole Condominium Association, laying out his grievances and alleging damages of more than $290,000 as of May. The association and EJF denied causing the damage, saying any damage was caused by third parties beyond their control and that Lambert failed to limit his losses. Neither immediately responded to requests for comment.

Police said Lambert carried out the shooting inside the Vida Fitness center on the ground floor of the building, which caters to residents. One woman said she was on a treadmill when she heard the shots and had to evacuate past the scene where one victim was killed. "It was like chaos. It was horrific, honestly," she said.

A witness who was about 20 feet from the shooting told CBS News he lived directly below Lambert and described him as increasingly upset at HOA meetings. He said Lambert used to invite people into his apartment to show them how bad conditions were, and he called the building, which has 99 units, a "tight-knit community." The witness said his girlfriend barricaded herself in their apartment during the standoff, and the unit is now under an inch of water from the response to a fire at the scene.

Logan Circle is a vibrant Washington neighborhood that gentrified in the 2010s, known for historic architecture and some of the city's most popular restaurants and bars. According to his LinkedIn profile, Lambert was the founder and CEO of MLEM, a healthcare and technology consultancy, earned his undergraduate and medical degrees at Marshall University in West Virginia, and completed his residency at West Virginia University.