The investigation is ongoing, police said. They would not release the officer's name.
Police said they determined that the victim committed suicide between 5:56 p.m. Sunday and 6:30 a.m. Monday.
"It is with deep regret that the Nassau County Police Department determined the deceased was a 25-year member of the department," a police news release said.
Deputy Insp. Kenneth Lack would only confirm the man was a patrol officer in the Eighth Precinct, based in Levittown.
Monday afternoon, uniformed Nassau officers could be seen inside the suicide victim's house, and one of them said the family did not want to speak.
Earlier in the day, police said the passerby who called 911 reported the body of a man hanging from a tree.
Yellow crime scene tape cordoned off the area late Monday morning and more than two dozen police cars were at the park, as police investigators processed the scene.