NEW YORK (AP) — An unidentified man could be responsible for four separate stabbings that occurred within a few hours in the New York City subways, leaving two people dead, authorities said.

The assaults happened between Friday morning and early Saturday, New York Police Department officials said during a news conference.

Authorities believe all four victims were homeless.

Police said they were searching for a man they suspect could be behind all of the attacks. They were reviewing subway security camera footage as part of the investigation. All four stabbings occurred along the A subway line.

One of the victims was discovered dead on a train in Queens at about midnight on Friday with several stab wounds to his neck and torso. Two hours later, a 44-year-old woman was found stabbed to death in the subway in upper Manhattan.

The two nonfatal attacks — one involving a 67-year-old man and the other involving a 43-year-old man — also occurred in upper Manhattan.

In response, NYPD officials said they would deploy an additional 500 officers into the subway system throughout the city as a safety measure.

“To the victims, to the victims’ families, we are 100% committed to getting justice … to bring closure to the families of this terrible incident,” NYPD Commissioner Dermot Shea said during the news conference.