Washington, D.C. police have confirmed a sixth person was wounded in Thursday's mass shooting at a Logan Circle fitness center, as investigators laid out new details of the hour-long barricade and fire that ended with the gunman — identified as a doctor who lived in the building — dead by suicide.

Police identified the shooter as 55-year-old Matt Lambert, a resident of an apartment building in the 1500 block of 15th Street NW. Investigators say Lambert first opened fire inside the VIDA Fitness gym, killing the building's manager, 46-year-old James Stewart, before fleeing to barricade himself inside his seventh-floor condo, according to NPR.

A maintenance dispute turned deadly

While barricaded, police say Lambert started a fire inside his unit and shot at least five more people from his balcony before negotiators' roughly hour-long effort to talk him into surrendering ended with him taking his own life. One of the wounded was taken to a hospital in critical condition after being shot in the head; the rest of the injuries were described by police as non-life-threatening, per CBS News.

Investigators believe the shooting was rooted in an ongoing dispute between Lambert and his condo building over mold and maintenance issues, according to CNN. The attack unfolded just before 8 a.m. Thursday, roughly a mile north of the White House, sending shockwaves through the Logan Circle neighborhood and prompting a heavy police response.

Police have not said whether Lambert will face posthumous findings or released the identities of all six gunshot victims. The investigation remains open.