NEW YORK — A 21-year-old Brooklyn man was arrested in the fatal stabbings of two people on New York City subway trains, police said Sunday.

Rigoberto Lopez was taken into custody Saturday night and was formally arrested Sunday on charges of murder and attempted murder, police said.

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

Two nonfatal attacks — one involving a 67-year-old man and the other involving a 43-year-old man — also occurred in upper Manhattan. Authorities believe all four victims were homeless.

Deputy Chief Brian McGee said officers on patrol in the northern Manhattan neighborhood where three of the four stabbings occurred spotted Lopez, who fit the description of the suspect being sought in the attacks, at 6:15 p.m. Saturday.

The officers brought Lopez to the local police precinct for questioning, McGee said. A knife was recovered from Lopez and will be analyzed to determine whether it was the weapon used in the attacks, McGee said.

Police said they believe Lopez may have committed all of the attacks. It wasn't clear if he had an attorney who could speak for him.

Police leaders said they would deploy an additional 500 officers into the subway system to guard against future assaults. "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 Saturday.

Transit officials called the 500 extra officers "an important first step" while asking for another 1,000 officers to be deployed to subways and buses to stem the violence.

"We request teams of uniformed officers be assigned to every station and that officers ride the system throughout the day and during the overnight to ensure the safety of our customers and colleagues," Metropolitan Transportation Authority CEO Patrick Foye and New York City Transit interim president Sarah Feinberg said in a letter to Shea and Mayor Bill de Blasio on Sunday.

Foye and Feinberg said that, "The safety and security of our system remains our foremost priority. We know you share this commitment."