The victim was cleaning a public bathroom when a man rushed in and attacked her.
It happened at St. Andrew's Playground in the Bedford-Stuyvesant section around 9 a.m. Thursday morning.
At that time, there was a park full of people, but no hint of a scream or distress.
"I think if we had heard her screaming a lot of us would have gone over there. I'm not the only guy that feels that way. We try to keep a safe neighborhood," said Robert Shaw, an eyewitness.
Police say a 29-year-old Parks Department employee couldn't cry out because her rapist covered her mouth.
As Robert Shaw played basketball at St. Andrews Playground in Bedford-Stuyvesant, he watched a crime scene go up around him, with white and blue tape, a swarm of officers, and an innocent victim.
"I seen her coming out on the stretcher, she was over there, all bloodied up. The police came. The ambulance came," Shaw said.
As the custodial worker started her shift, police say the suspect approached her from behind in a utility closet near the restrooms and closed the door behind him.
He started to beat hit her, and pulled out either a knife or box cutter, and then raped her.
Sources say this park employee stopped the rape when they knocked on the door twice, and startled the rapist, who then opened the door and fled on a bike.
The victim's fiancé showed up at the scene about two hours after the incident.
"It could have been me in there. And all this the time I'm thinking it's women coming in there, and then I seen those guys last week, I felt strange about it," said Wanda Robinson, a Brooklyn resident.
Investigators worked this case hard and spent the day canvassing the neighborhood for clues.
"It's sad. The park is supposed to be a place where you can enjoy yourself," said Louise Tatum, a Bedford-Stuyvesant resident.
"It's an eerie feeling. This is a park, we come here for recreation. Instead we see trauma," Shaw said.
So far, no arrests have been made. Police released a sketch of the suspect on Friday.
The victim is being treated at an area hospital.