New York - Detectives have solved the decades-old mystery of "Baby Hope," a little girl whose body was discovered inside a picnic cooler beside a Manhattan highway in 1991, and on Saturday arrested a cousin of the child after he admitted he had sexually assaulted and smothered her, police said.

Conrado Juarez, 52, was arrested on a murder charge and was awaiting arraignment, Police Commissioner Raymond Kelly said. He was 30 at the time of the girl's death.

It wasn't clear whether Juarez had a lawyer. Police said he lived in the Bronx, but that the family had been living in Queens at the time of the killing. They also said Juarez claimed that a relative had helped him dispose of the girl's body.

Anjelica Castillo's naked, malnourished corpse was discovered on July 23, 1991, alongside the Henry Hudson Parkway. Detectives thought she might have been suffocated but had few other clues as to what happened to her.

The case became an obsession for some investigators. Hundreds of people attended a funeral for the unknown girl in 1993. Her body was exhumed for DNA testing in 2007, and then again in 2011.

In July, on the 22nd anniversary of the discovery, detectives tried another round of publicity. They canvassed the neighborhood where her body was found, hung fliers, circulated sketches of the girl and a photograph of the cooler and announced a $12,000 reward for information leading to an arrest.

Former Detective Jerry Giorgio, who had the case from 1991 until his retirement this summer, said he remained confident the case could be solved. Assistant Chief Joseph Reznick, who also worked the case, said they never gave up.

"I think reflecting back on what we named this little girl, Baby Hope, I think it's the most accurate name we could have come up with," he said.

Giorgio left the NYPD and went to the Manhattan district attorney's cold case squad, from which he retired this year. "I missed the tipster call by a couple of weeks, damn it," he said.

A tipster led police to Anjelica's sister, who told detectives that her sister had been killed. Police matched DNA from Anjelica to their mother. The mother, who was not identified, didn't have custody of Anjelica at the time of the girl's death - she had been living with an aunt on the father's side, Bavlina Juarez-Ramirez, police said.

Police closed in on Juarez and waited for him Friday outside a Manhattan restaurant where he worked as a dishwasher. He told them he had killed the girl at the apartment of his sister, Juarez-Ramirez.

"When she went motionless, he summoned his sister from another room," Kelly said.

Then, the sister, who is now dead, got the blue cooler and helped dispose of the body. Kelly said they took a livery cab from Queens to Manhattan where they dumped the cooler, then separated.

The cooler, which held the girl's remains and unopened cans of Coke, was later discovered by construction workers.

Kelly called the arrest a superb case of detective work, and said he was proud of his officers. "For me, it makes you proud to be a member of this organization - they were unrelenting."

Juarez was being held on murder charges and was awaiting arraignment.

The detectives assigned to the case were instrumental in organizing a burial in a Bronx cemetery for the girl in 1993. Hundreds attendedl; Reznick gave the eulogy. The girl was dressed in a white frock and buried in a white coffin.

The detectives paid for the girl's headstone that reads: "Because we care."

On the tomb sit two little angels.