One month after Amy Winehouse was found dead in her Camden, London, home, the singer's passing still remains a mystery. A toxicology report released this week by Winehouse's family reveals that no illegal substances were found in the Grammy-winner's system when she died, seemingly debunking a theory that the singer — famous for her drug addictions — had overdosed. Suggestions that her death was related to alcohol withdrawal have also been cast in doubt, given that traces of alcohol were found in her body. Still, that's hardly conclusive. Here, 4 potential causes of death:

1. Emphysema
Back in 2008, Winehouse's father, Mitch, revealed that his daughter had been diagnosed with early-stage emphysema, says Amina Khan at the Los Angeles Times — "a rare disease in the twenty-something set," likely caused by the singer's habit of smoking crack cocaine and cigarettes. Doctors advised her that the condition could be fatal, especially if she continued her dangerous lifestyle. At the time, Winehouse's father also said that she suffered from an irregular heartbeat. It's possible the combination of the two ailments killed Winehouse.

2. Bulimia
The singer struggled with chronic eating disorders, says Zara Bokhari at the International Business Times, and dramatic weight loss before her death suggests that she may have been battling bulimia again. "We may be dealing with a [death caused by] dehydration," says forensic pathologist Cyril Wecht, as quoted by ABC News. Emaciation can throw off a bulimia sufferer's "metabolic fluid balance and electrolytes."

3. Alcohol withdrawal
The theory that Winehouse died as a result of detoxing from alcohol too quickly still has merit, hazards Khan. Health experts say that quitting cold turkey without medical counsel could cause a stroke or heart attack in anyone dependent on booze. Even though alcohol was detected in her system, perhaps it was "too little." I don't see it, says addiction specialist Jeremy Clitherow, as quoted by the Chicago Tribune. "It is very unusual to die from alcohol withdrawal... you can get delirium tremens (DTs), but normally you survive."

4. Prescription drugs
We may not be getting the whole story, says Eric Zorn at the Chicago Tribune. When Winehouse's family revealed the details of the toxicology report, they specified only that no illegal drugs were found in her system. This opens up the possibility that legal substances were involved, says Wecht. "Most drug deaths are from legally obtained drugs." Furthermore, says toxicology professor Bruce A. Goldberger, prescription drugs and "designer" drugs often escape detection in these reports.

Sources: ABC News, Chicago Tribune (2), International Business Times, LA Times, NY Times