A man was fatally stabbed and beaten with baseball bats inside a Brooklyn building early Monday, police said.

Police found the 20-year-old victim lying in the lobby of a building on 16th Street near Seventh Avenue in South Slope with a stab wound to his chest and severe trauma all over his body around 4 a.m.

He was rushed to Methodist Hospital where he was pronounced dead, police said.

It’s believed three unidentified men attacked the victim in the lobby where they stabbed him and beat him with bats, authorities said.

Police are questioning a person who witnessed the brutal killing but said that person is not a suspect.