Authorities said the attack happened just before 9 a.m. inside a second-floor apartment in the 200 block of S. Carondelet Street. Police said the boy was stabbed several times. He was taken to the hospital, where he was pronounced dead.
"They were taking the little boy downstairs and he had a lot of blood on his stomach, his hand and the upper-right part of his head," said neighbor Erica Hernandez. "They were trying to revive him as they were walking him out, but he looked like he was in really bad shape."
A 26-year-old man, who was in a romantic relationship with the boy's mother, was taken into custody. Police said the man was stabbed in the shoulder during the attack. Neighbors said he was wounded by the boy's mother as she tried to protect her son.
A kitchen knife believed to be the murder weapon was recovered at the scene.
A neighbor who did not want to be identified said the floors of the apartment were covered in blood and much of the furniture had been damaged.
Neighbors said they had heard an argument coming from the apartment and the boy calling for help.
Police said the attack appeared to be a case of domestic violence. The cause of the argument was not yet known.