Lt. Kevin Smith, a detective with the Nassau County police, said the fire was in the stairwell of the two-story building in Lawrence on Long Island, the New York Post reported. He said the fire marshal called the blaze "suspicious."
"They have eliminated all natural and accidental causes and it is being investigated by our homicide squad," Smith said.
Marena Vanegas, 46, was trapped in the building and died with her son, Saul Presa, 20, and daughters Andrea Vanegas, 13, and Susanna Vanegas, 9. Her husband and two sons jumped through a second-floor window.
Witnesses said Eddie Vanegas stood outside begging his wife to throw the two girls through the window and to jump herself, Newsday reported.
A 12-year-old son, also named Eddie, said he and his brother, Lionel, 9, and their father had been given shelter by a church.
A family living in the other apartment in the two-story building also got out through windows.