The altercation started on a northbound E train approaching the West 4th Street Station on June 24 around 3:30 p.m.
The disturbing encounter all started when a father on the train was sitting with his four-year-old daughter and wife and observed the suspect hit his crying baby.
The man, identified as "Robson" then told the suspect "you shouldn't do that!"
"He hit his child on the arm. Because it was silent at that time, everybody could hear the noise. He hit his child so hard," Robson said.
Sources say the suspect then told Robson to worry about his own kid, saying, "Come do something! I will kill you. I want you to do something."
Robson then tried to calm his crying daughter by telling her, "It is going to be okay."
He then says passengers tried to intervene and that's when the suspect turned violent, cursing wildly at everyone.
Police say the suspect then said, "It will be okay. Your daddy will get killed on this train (expletive.)"
The suspect kept the train car doors open and displayed a knife before spitting into the train car. He also reportedly called his own son a f***.
The suspect is described as a black man in his twenties with a red t-shirt. Police want to make sure his child is okay.
Anyone with information in regards to this incident is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).