>>> 22 years since new york city detectives vowed to find the killer of a young girl whose body was found stuffed in a cooler on the side of the highway. she became to be known as baby hope. but her real name is finally known and her alleged killer under arrest.

>> reporter: after more than two decades, the murder case of a young girl known only as baby hope until now is finally solved. police say following the arrest of a suspect on saturday.

>> today, nypd investigators have given young anjelica her due justice.

>> reporter: in 1991 , police found the body of a 4-year-old girl in a picnic cooler, dumped along a highway in manhattan. it launched an investigation that would span 22 years. officers assigned to the case, including retired homicide detective giorgio never stopped looking for her killer.

>> we thought of her as almost one of our own children.

>> reporter: they helped to pay for her head stone at saint raymonds cemetery, enkriebed because we care. this sum isser they renewed their focus on the cold case , canvassing neighborhoods for clues. in july, a woman came forward, investigators say, with information about the child's adult sister. the tip and dna test allowed the police to locate the baby's mother and identify a suspect. saturday, police arrested cored in a doe juarez , a cousin of baby home, now identified as anjelica castillo.

>> juarez admitted he sexually assaulted anjelica. that he smothered her and disposed of the body. with the aid of juarez ramirez who is now deceased.

>> reporter: detectives who worked the case from the beginning expressed relief.

>> when we vids this plot now out at saint raymonds we can now attach a name to this little girl .

>> reporter: a name and a place in the hearts of those who never gave up. michelle franzen , nbc news, new york.