The Oscar-winning actor was found unconscious on his bathroom floor by screenwriter David Katz and another friend on Sunday at about 11:15 a.m. ET. NYPD investigators found roughly 70 dime bags of heroin inside the apartment -- 5 were empty and 65 were unused. Used syringes and drug paraphernalia were also found in Hoffman's apartment, according to ABC News.
While detectives try to track the heroin's origin, narcotics officers are cross-checking the stamps on the bags. Investigators are looking into the purity of the heroin and whether there were any additives, law enforcement officials told ABC News.
Hoffman was supposed to pick up his three children on Sunday but never showed up. The 46-year-old actor was last seen Saturday at 8 p.m.
"We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support we have received from everyone. This is a tragic and sudden loss and we ask that you respect our privacy during this time of grieving. Please keep Phil in your thoughts and prayers," Hoffman's family said in a statement.
While it appears the actor died from a drug overdose, the official cause of death has not yet been determined. Autopsy results were expected Monday afternoon, but the medical examiner's office said it continues to examine the cause, and the results are not ready.
Hoffman had struggled with drug abuse since college, friends said. In a 2006 interview with "60 Minutes," Hoffman said the problem peaked in his 20s before he sobered up.
"I was 22, and I got panicked for my life," he said in the interview.
After more than 20 years of sobriety, Hoffman admitted last year that he relapsed and did another stint in rehab after using heroin. Hoffman's death has highlighted a growing problem in the United States. The Drug Enforcement Administration says heroin overdoses have increased 45 percent from 1999 to 2010.
Broadway theaters will dim their marquees for one minute at 7:45 p.m. ET Wednesday in Hoffman's memory.
Hoffman appeared in 50 films in less than 25 years. He was nominated for an Oscar for his performances in "The Master," "Charlie Wilson's War" and "Doubt,"and won an Oscar for his leading role in the 2005 film "Capote." He also appeared in films such as "The Big Lebowski" and "Almost Famous" and "The Hunger Games: Catching Fire," as Plutarch Heavensbee.
Hoffman is survived by his longtime girlfriend, costume designer Mimi O'Donnell, and their three young children.
ABC News and The Associated Press contributed to this report.