A suspect has now been arrested and charged with murder in connection with multiple stabbings that left two victims dead on the New York City subway system within a 24-hour period, police said.
Rigoberto Lopez, 21, from Brooklyn, is facing one count of first-degree murder, two counts of second-degree murder and two counts of attempted murder, police said.
The incidents all occurred along the A subway line. At least three of the stabbings appear to be connected, and police were investigating whether the fourth is as well, NYPD Commissioner Dermot Shea said earlier Saturday during a press briefing.
The first incident occurred Friday at approximately 11:20 a.m., when a 67-year-old man was stabbed by an unknown man at the West 181st Street station in upper Manhattan, police said. He was treated at an area hospital and is recovering.
Later that day, shortly after 11 p.m., a man was found on the A train at the Far Rockaway-Mott Avenue station in Queens with stab wounds to his neck and torso, police said. EMS arrived and pronounced him dead at the scene.
Two hours later, at approximately 1:15 a.m. Saturday, an MTA employee found a 44-year-old woman unconscious on the train at the 207th Street station in upper Manhattan with multiple stab wounds, police said. She was pronounced dead at a nearby hospital.
The fourth incident occurred back at the West 181st Street station in Manhattan, where a 43-year-old man was stabbed shortly before 1:30 a.m. Saturday, police said. He underwent surgery at a nearby hospital and was in stable condition.
All of the victims appear to be homeless and the stabbings were unprovoked, police said. The three late-night incidents are believed to be connected, police said.
Detectives stressed that the investigation is preliminary, and that they are trying to determine definitively if all four stabbings were committed by the same person.
In light of the deadly violence, the NYPD will immediately deploy an additional 500 officers throughout the city to patrol the transit system above and below ground, Shea said.
"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. "We want to do everything we can to make sure it remains the safest system, and that people also feel safe."
The heads of the city's transit system and transit workers union called the attacks "outrageous and unacceptable."
"Every customer, and each of our brave, heroic transit workers deserve a safe and secure transit system," New York City Transit interim President Sarah Feinberg and TWU Local 100 President Tony Utano said in a joint statement. "We have been calling on the city to add more police to the system, and to do more to assist those who desperately need mental health assistance. The time for action is now."
ABC News' Aaron Katersky contributed to this report.