The person behind the attacks, police said, was Kenny Alexis, a homeless man who was arrested on charges including attempted murder, assault and criminal possession of a weapon.
Alexis, 21, was taken into custody without incident around 4:15 a.m. Wednesday outside a fast-food restaurant in midtown Manhattan shortly after the two Montreal women, ages 22 and 25, were stabbed, Police Commissioner Ray Kelly said.
The police department's top spokesman, Paul Browne, said Alexis admitted to stabbing the Canadians, a Brooklyn man and a Texas man. The latter two were ambushed in separate subway attacks. Browne also said witnesses identified Alexis in all the attacks but one.
Alexis was carrying the folding knife used in the attacks on the two Canadians when he was apprehended, Kelly said.
Three of the victims are still in the hospital.
The father of Christopher McCarthy, 21, of Houston, who police say was stabbed on a subway car on Manhattan's Upper West Side, said during a news conference that his son had forgiven the assailant.
"He hopes that (the attacker) can get help," Joe McCarthy said.
Police said Alexis, who was living in a West Side men's shelter, was arrested previously in Boston and New York on a number of charges, including attempted assault, criminal trespassing, disorderly conduct, shoplifting and criminal mischief.
Police said they did not see any signs that Alexis is mentally ill. Kelly said there has been no evidence to suggest that Alexis targeted tourists and police are still determining his motive.
The attack on McCarthy drew comparisons to the 1990 death of 22-year-old Brian Watkins, a Utah tourist stabbed to death in a Manhattan subway station while defending his mother during a robbery.
Kelly said Wednesday that the subway system, which carries millions of people each day, has never been safer.
The slew of stabbings comes on the heels of recent news that violent crime as a whole in the city continues to decline despite a national spike. The FBI has said that while homicides rose 4.8 percent nationwide last year, they fell 5.4 percent between 2004 and 2005 in New York City.
Violent crime in New York dropped 1.9 percent, according to the FBI, in a year when such crimes rose 2.5 percent nationwide.