The wealthy doctor who opened fire inside a Washington, DC, gym barricaded himself in his mold-filled penthouse for nearly an hour — and called a neighbor to gripe — before ultimately killing himself in a standoff with cops, according to a federal law enforcement source.

During the tense and bizarre confrontation, emergency room doctor Matt Lambert holed up in his $1.1 million abode shortly after killing at least one person and injuring four in the ground-floor gym of his trendy Logan Circle building.

Lambert first called a neighbor and admitted to the shooting before inexplicably setting off the building’s sprinkler system, according to the law enforcement source.

Metropolitan Police Department negotiators then called Lambert, who launched into a tirade about his homeowners association — which he’d been battling for years over black mold.

Shortly after, he shot himself inside his 7th-floor home, the law enforcement source said.

Lambert killed one person and left four others injured in Vida gym, where he is believed to have been a member, according to police.

Lambert filed a lawsuit against his homeowners association in March 2025 over the dangerous fungus, and cited it in a “manifesto” he posted on X shortly before the shooting.