A man fatally slashed his girlfriend, then slit his own wrists in what cops are investigating as a murder-attempted suicide inside the couple’s Brooklyn apartment, authorities said Tuesday.

Officers responded to a 911 call around 5:10 a.m. for an assault inside the fifth-floor apartment on Lefferts Avenue near Sterling Street in Prospect-Lefferts Gardens.

Investigators suspect the 58-year-old man — identified by police sources as Romeo Borneo — pulled a knife on his 47-year-old girlfriend, identified as Erica Renaud, slashing her in the neck and killing her. He then slit his own wrists, police said.

Renaud, a cafeteria supervisor at NYU, was pronounced dead at the scene. Her body showed signs indicating it had been dead for several hours prior to being discovered on Tuesday, police sources said.

A knife was also recovered inside the apartment, where both victims lived, cops said. No one else was believed to be inside the apartment at the time of the incident.

Her beau was rushed to Kings County Hospital in serious condition. He was not immediately charged.

“I’m numb,” said Tony Joseph, 54, who said he was like a brother to Renaud. “I’m numb. I just don’t want to believe it even though I see it – don’t want to believe it.”

He said his cousin called him to report that there were police in the building.

“She told me she saw the cops rushing upstairs,” Joseph said. “She told me the guy who lived with a girl in the building just killed her. I don’t know, my instinct just say….I called [Erica’s] phone, it rang and she didn’t answer, and she always answered her phone, and that’s why I flew over here.”

Renaud had been saving money to move out of the home where she was staying with Borneo, Joseph said.

Borneo’s aunt, Gloria Burnett, claimed that Renaud had been cheating — and “threatened [Borneo] everywhere – on the street.”

“She would [be] embarrassing him to the fullest, to the fullest that he can’t take it no more,” she said.

But Joseph shot back against those claims.

“Even if she was, even quote was cheating that doesn’t give you the right to take somebody’s life,” Joseph said. “Leave. Put her out.”

Police were previously called to the home for a domestic dispute in 2016, though details of the incident weren’t immediately available.