D.C. Shooter Killed At Least One Person and Himself, Identified as ER Doctor

The man who shot several people in D.C. killed one person and later himself has been identified as an emergency room doctor ... who was locked in a dispute with his homeowners' association.

The shooter, Dr. Matt Lambert, opened fire Thursday morning in the VIDA Fitness gym next door to his building, where he killed 1 person and injured 4 others.

He reportedly then went upstairs and barricaded himself inside his 7th-floor apartment before killing himself.

Lambert had sued Metropole Condominium Association last year, claiming he had black mold in his million-dollar apartment that he believes was caused by repairs to the building.

According to the New York Post, an X account called OfMiceandMold -- reportedly connected to Lambert -- had been documenting his ongoing battle with the HOA.

The account shared a chilling post Thursday where he wrote ... "My manifesto is an engineering report" and included screenshots of the document.

D.C. officials say they're investigating the ongoing battle with the HOA as a possible motive for the deadly shooting.

Ward 2 Council Member Brooke Pinto told NYP that information seems to suggest Lambert had been targeting someone in the gym he' "been in active litigation with over a condo issue."

She said ... "It’s just so sad and so senseless to kill someone over a dispute that should be worked out reasonably."