NEW YORK (AP) — The "preppie killer" who served 15 years behind bars for strangling a woman in Central Park during what he said was rough sex is headed back to prison for a drug offense.

Robert Chambers pleaded guilty Monday to criminal sale of a controlled substance and assault on a police officer. He and his girlfriend were arrested in October 2007 in an undercover sting at their Manhattan apartment on charges of dealing cocaine.

The district attorney's office said Chambers has been promised 19 years and four months in prison when he is sentenced next month. Chambers will get six years on the assault charge, which will run concurrently, and will have five years of supervision after his release.