Hoffman, a 46-year-old father of three who was considered one of the finest character actors of his generation, was found lying on his apartment bathroom floor with a needle still stuck in his arm.
Empty and full bags of heroin were found in the apartment and while the autopsy will confirm the precise cause of death, few expect any announcement other than an overdose.
"Heroin is a growing epidemic," US Drug Enforcement Administration (DEA) spokesman Joseph Moses told AFP.
US heroin-overdose deaths rose by 45 percent from 2006 to 2010, and the amount of heroin seized each year on the Mexican border was up nearly four times from 2008 to 2012, he said.
"First time users are younger than they were years ago and it's not in the cities anymore, it's gone into rural areas, into suburbia," Moses said.
Hoffman is now the second high-profile actor in months whose death has been linked to the class A drug.
Cory Monteith, the 31-year-old Canadian star of hit TV series "Glee", died of an accidental heroin and alcohol overdose in a Vancouver hotel room last July.