UPDATE: 4:13 p.m.

CHICAGO (CBS) -- A 48-year-old Chicago area man has been freed from prison after 29 years - now that the Cook County State's Attorney has dropped the charges against him for the 1984 murder of a teenage girl.

At the center of this: the murder of 15-year-old Kristina Hickey of Park Forest.

Her body was found in the fall of 1984 - and another teenager, Christopher Abernathy, was arrested and then convicted - and sentenced to life in prison.

Cook County State's Attorney Anita Alvarez says last year, DNA evidence showed Abernathy was not a match.

So now, she says, the killer of Kristina Hickey has still not been identified.

"Kristina's mother and her family have lived through a nightmare, and they are probably suffering deeply again today as a result of my decision.

"But I cannot and I will not let a wrongful conviction stand. And in our assessment, Mr. Abernathy's conviction was just that."

Back in the '80s, Abernathy signed an admission of guilt - which Alvarez called "thin," since Abernathy has a dinished mental capacity.

Now Alvarez says - they'll try to find the real killer.