The five young adults were headed to work at a camp for people with disabilities when they crashed.
Thursday night, hundreds crowded Our Lady of Victory for mass, while hundreds more spilled onto the sidewalks in Floral Park.
Police say the dark Honda sedan was traveling south on the parkway around 8:45 a.m. when it veered off the right side of the road and crashed into a tree.
"These were three young adults doing something positive for the community," state Police Major Walter Heesch said in a statement.
The crash was so violent that rescue workers had to cut off the roof off the car to pull the victims out.
The group was driving en route to Camp Anchor, a town-run seaside camp for about 600 adults and children with disabilities ranging from quadriplegia to autism to Down syndrome, said Hempstead Town Supervisor Kate Murray.
"We are all devastated by this tragedy," Murray said.
"Alcohol involvement has been ruled out by State Police investigators," Heesch said. "Our prayers go out to the families in this tragedy."
The six-week camp's name is an acronym for Answering the Needs of Citizens with Handicaps through Organized Recreation. It has one counselor for every camper, Murray said.
Officials restricted access to the camp in Lido Beach, on the southern shore of Long Island. They cited the need for privacy of both the campers and counselors, who Murray said were devastated. A police car was posted at the gate.
"As anybody who's ever come to Camp Anchor knows, the heart and soul of the camp are the young men and women who mostly volunteer and work with our campers," Murray told reporters outside the facility. "There is a very special bond that is life-enhancing both for the campers and the counselors."
Although many young people volunteer at the camp, the crash victims were paid employees who have worked with the summer program for three or four summers.
The campers had varying degrees of reaction, depending on their abilities, Murray said. But "they know that a big tragedy has happened today," she said.
(The Associated Press contributed to this report)