WASHINGTON — A 13-year-old Virginia girl was most likely stabbed to death by a Virginia Tech student on the day she disappeared, the authorities in Blacksburg, Va., said Tuesday, and a second university student — originally accused of helping dispose of the body — now faces charges of helping commit the murder.

The girl, Nicole Madison Lovell — a liver-transplant and cancer survivor — disappeared last week from her home in Blacksburg, setting off an intense search as well as outrage over the role social media might have played in the case.

Mary Pettitt, the commonwealth’s attorney for Montgomery County, Va., announced the “very preliminary” cause of death — based on the early results of an autopsy — and the additional charges at a brief news conference. Nicole’s mother, Tammy Weeks, broke down in tears at the conference while speaking of her daughter, who had grappled with serious illness since she was born and needed daily medication to survive.

Nicole, called Coley by her family, received a liver transplant when she was 10 months old to treat a “rare tumor” in her liver, her mother said. Then, at 4, she received a diagnosis of and was treated for non-Hodgkin’s lymphoma, a type of cancer. She had acute respiratory illness and slipped into a coma for six months, and then developed a potentially deadly bacterial infection.

“We were advised at this time that she had only a 1 percent chance of survival,” Ms. Weeks said, adding, “Coley once again beat the odds.”