The first attack happened shortly before midnight on Friday, when an adult male was pronounced deceased after being stabbed in the neck and torso on the A train.
"To the victims, to the victims' families, we are 100 per cent committed to getting justice ... to bring closure to the families of this bad incident", NYPD Commissioner Dermot Shea said during the news conference. About two hours later in the early hours of Saturday, an unconscious 45-year-old female with multiple stab wounds was transported to a local hospital where she was pronounced deceased.
Detectives said the attacks, all on the A train or at stations served by it, were unprovoked and that they are investigating whether they were all committed by the same person. Police are searching for a man they suspect could be behind all of the attacks. After undergoing surgery, he was in a stable condition, police said.
The department has deployed an additional 500 officers into the subway system throughout the city as a safety measure.
Investigators said it's unclear whether the stabbings were connected and stressed the investigation is preliminary.
"I know what the train used to look like, and when you look at what the train looks like now and you look at how far crime has fallen over the years, but we don't want to go one step back", Shea said.
The heads of the city's transit system and transit workers union called the attacks "outrageous and unacceptable".
"Three of these incidents appear to be connected and the detective bureau is looking into the possibility that all four could have been committed by one individual", said NYPD Chief of Transit Kathleen O'Riley said.
In the meantime, Shea said the deployed officers will be there as long as needed to ensure that New Yorkers feel safe. "The time for action is now".