All four stabbings occurred along the A subway line.
A 67-year-old man and a 43-year-old individual were also stabbed on the A train in the last 24 hours, though their injuries do not appear to be life-threatening, authorities said. That victim is recovering in the hospital.
An unidentified man was pronounced dead on the scene after suffering stab wounds to the neck and torso. Two hours later, a 44-year-old woman was discovered unconscious with stab wounds on an A train at the 207th Street station in Manhattan. She was pronounced dead at a nearby hospital.
The officers brought Lopez to the local police precinct for questioning, McGee said.
Brown declined to say when the person was taken into custody, or whether the person was a suspect in one or more of the four unprovoked attacks that apparently targeted homeless people and took place on the A train or at stations served by it beginning Friday morning. Police are searching for a man they suspect could be behind all of the attacks.
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.
The announcement comes after the New York Police Department (NYPD) deployed an additional 500 officers Saturday to patrol above and below ground within the subway system Saturday in response to the seemingly unprovoked attacks all happening along the A train.
"Every customer, and each of our fearless, 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.
In the meantime, Shea said the deployed officers will be there as long as needed to ensure that New Yorkers feel safe. The attacks came in the city's latest string of subway crimes. "The time for action is now".