Police ruled Hannah Graham's death a homicide Tuesday in the first official confirmation that the University of Virginia sophomore was murdered in September. Investigators from Albemarle County and Richmond said Graham died of "homicidal violence" but did not provide any further information. The autopsy report has not yet been released, USA Today reported

"The Albemarle County Police Department and the Albemarle County Commonwealth’s Attorney remain committed to the investigation. To protect the integrity of the case, we will not be answering questions or releasing additional information," the police said in a statement. "We are working hard to ensure that justice is served."

An administrator at the Richmond Medical Examiner's Office told the Washington Post that investigators knew how Graham died, but police had asked them not to release the details.

The homicide classification comes about a month after the remains of 18-year-old Graham were found on an abandoned farm. She had disappeared Sept. 13 after partying with friends, getting lost near the university in Charlottesville, and walking into a downtown mall around 1 a.m.

Security camera footage filmed Graham walking after 1 a.m. with Jesse Leroy Matthew Jr., a 32-year-old university medical center employee. Matthew was charged Sept. 23 with abduction with the intent to defile -- a class two felony -- but not with her murder. He was due in court Dec. 4.

Last week, Matthew pleaded not guilty to another set of charges from 2005. DNA evidence connected him to a sexual assault, attempted abduction and attempted murder in Fairfax County outside Washington, D.C. That trial was set to start March 9. Matthew also may be involved with the death of Virginia Tech student Morgan Harrington in 2009, but no charges have been filed, the Cavalier Daily reported.

Matthew's lawyer, James L. Camblos III, said the homicide determination in Graham's case was not a shock. "The findings were not unexpected," Camblos told the Washington Post.