A Long Island man accused of stalking and gunning down his ex-girlfriend in a Northwell Health parking garage hit the gym less than an hour after the killing, authorities said.

Quay-Sean Renard Hines, 30, pleaded not guilty to second-degree murder charges at his arraignment Saturday in the March 31 shooting of Amelia Laguerre, a longtime Northwell Health administrative assistant.

Less than an hour after unloading 10 shots at Laguerre in the dimly lit parking structure underneath a Northwell’s medical complex in New Hyde Park — hitting her nine times — Hines went to an LA Fitness in Levittown, Nassau County District Attorney Anne Donnelly said at a news conference, calling it a “particularly callous case.”

Laguerre, the mother of a 5-year-old son, told family and friends that Hines was mentally and physically abusive towards the end of their two year relationship, police said.

Hines burned her clothes, damaged her property and broke some of her child’s belongings before they broke things off late last year, according to cops.

Police said they suspect that Hines broke into Laguerre’s apartment in October 2021. He also began to stalk her, they said. Hines would sometimes show up at her work, trying to rekindle their relationship — but it would just devolve into heated arguments, they said.

“According to the preliminary investigation, less than one hour after Hines allegedly shot Ms. Laguerre, he went to the gym [LA Fitness in Levittown],” Donnelly said. “The murder was a selfish act. It was a calculated act. It was a cowardly act.”

Hines has no prior arrests or domestic incidents in Suffolk, Nassau or Queens and, despite trying to win Laguerre back, is currently in another relationship, police said.

“He does have other relationships,” Police Capt. Stephen Fitzpatrick said Saturday. “So if any of these females that are involved with him or had been involved with him that he had abused or had this kind of behavior towards, we ask them to come forward.”