What started out Sunday night as a suspected sexual encounter involving two men and a woman ended with one of the male lovers getting pounded with a baseball bat and stabbed to death.

The 20-year-old victim was discovered unresponsive in the Park Slope section of Brooklyn after sustaining at least five stab wounds to his chest and back.

Police were summoned to the scene at 355 16th street at around 4am and found the victim lying in the first-floor hallway of the apartment building.

The victim was rushed to Brooklyn Methodist Hospital, where he was pronounced dead about 30 minutes later. The NYPD was withholding his name pending the notification of family members.

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The New York Daily News reported, citing unnamed police sources, that the deadly violence had been preceded by an amorous encounter involving the 20-year-old victim, another man and a woman, both aged 21.

The group sex session reportedly ended on a sour note after one of the men, presumably the victim, started filming the ménage a trois, which made the female participant upset.

The 21-year-old man, who lived at the apartment on 16th Street, then drove the woman home to Staten Island. When he returned, the 20-year-old victim, who did not reside there, took off.

Around that time, the victim's girlfriend, who was not involved in the orgy, got a message on Instagram that reportedly read: 'your boyfriend f***ed up and will be taken care of,' according to the Daily News.

Sources told the paper surveillance video from the area captured two white men and a black man running after the victim trying to escape from the building.

Witness Mike Rivera, who lives on the first floor in the building on 16th Street, tells DNAInfo New York that he emerged from his apartment into the hallway after hearing a woman's screams and saw the victim bleeding profusely and looking pale.

Rivera said the 20-year-old was mumbling something incoherent before he collapsed.
Moments earlier, the mortally wounded man knocked on a neighbor's door begging for help, with blood gushing from his wounds. A relative of the woman who answered the door told the paper the 20-year-old apologized profusely before falling to the floor.

Police officers and crime scene investigators spent the morning hours interviewing residents and collecting evidence at the scene.

Neighbor Ros Hayes tells CBS New York he heard the sounds of partying coming from the back of the building at around 2.30am.

So far, no arrests have been made in the deadly stabbing, which police are investigating as a possible domestic incident.