Riverside Park along the Hudson River on the Upper West Side of Manhattan was filled with people jogging, walking, biking or simply sitting on benches.
But there was someone else lurking: a man the police described as emotionally disturbed, wielding scissors and looking for victims.
Acting without any apparent reason on Tuesday morning, the police said, he stabbed or slashed five people in less than 10 minutes, an outburst of violence all the more terrifying for its randomness.
The police arrested Julius Graham, 43, but only after attacks on two female joggers, a man out for a walk with his dog, and a father and the two-year-old son he was pushing in a stroller.
A bystander wrestled the attacker to the ground as police officers rushed in and placed Graham in custody, according to the authorities.
Graham was carrying identification from Texas, but he had been living in a homeless shelter in the Bronx as recently as one week ago, according to the police commissioner, Raymond Kelly.
The suspect was taken to Bellevue Hospital Centre for evaluation and remains in custody. Police said none of the victims was in critical condition.
D. J. Jaffe, the executive director of the Mental Illness Policy Organisation, said the attack in the park highlighted broader problems with how the city dealt with people with mental illness.