NEW YORK (REUTERS) - New York City police said on Tuesday they identified the mother of a girl dubbed "Baby Hope", whose body was found crammed inside a picnic cooler on the side of a highway more than 20 years ago.

The identity of the slain girl remained a mystery until police got a tip this summer from a woman who said she thought she knew Baby Hope's sister.

Police used that information to track down the slain child's mother and said DNA testing confirmed her identity. The woman's name has not been released. She is cooperating with the investigation, and no charges have been filed, police said.

Baby Hope's bound, asphyxiated body was discovered stuffed underneath cans of soda inside a blue-and-white cooler alongside the Henry Hudson Highway in northern Manhattan in July of 1991.

She was estimated to have been three to five years old, had been starved and sexually abused, police said.

New Yorkers for years saw police sketches of the dead child on posters around the city.

Members of the police department gave the unknown child the name Baby Hope and paid for her funeral. Many of them worked for years in hopes of identifying her or her killer.

She was never reported missing.

The break came after police in July combed the neighbourhood where the body was found yet again, hoping for a fresh lead.

Out of that effort, key information came in a telephone call to the police department's tip line, said Deputy Police Commissioner John McCarthy.

"The caller recalled a conversation with or among people in a commercial store, where a woman said that she had a sister who had died and that the story was very similar to that of Baby Hope," McCarthy told Reuters by telephone.