NEW YORK (CNN) -- A former New York City bouncer was sentenced to life without parole Wednesday for the brutal slaying of a graduate student from Boston, the Brooklyn district attorney's office said.



Darryl Littlejohn, 44, is already serving prison time for the attempted kidnapping of another student in 2005.



Darryl Littlejohn, 44, was convicted of first-degree murder last month in the 2006 death of Imette St. Guillen, 24, who had been studying criminology at John Jay College of Criminal Justice.



Littlejohn, who is already serving 25 years to life for the October 2005 attempted kidnapping of a 19-year-old Queens college student, will serve out the sentences consecutively, said Sarah McNaughton of the Brooklyn district attorney's office.



During the trial, witnesses said they saw Littlejohn and St. Guillen leaving The Falls bar in lower Manhattan together early February 25, 2006.



Littlejohn was working as a bouncer at the bar.



Hours later, St. Guillen's nude body was found in an isolated lot in Brooklyn. Her face was covered with strips of packing tape, and a sock was stuffed into her throat.



She died of asphyxiation, and investigators determined she had been raped.



Littlejohn was charged with murder after investigators linked his DNA to blood found on plastic ties used to bind St. Guillen's hands behind her back.



Littlejohn's attorney, Joyce David, has filed an appeal on behalf of her client and maintains his innocence. She said that although Littlejohn has a long criminal record, he has no history of violence against women.



The horrific incident spread shockwaves through New York City nightlife, which relies heavily on unlicensed bouncers to keep order in the city's bars and nightclubs.



Littlejohn is being held at New York's Rikers Island maximum security facility.



CNN's Chris Kokenes and Kristen Hamill contributed to this report
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