No one will know till they get those toxicology reports back from teh post-mortem.
Everything else is just rumour.
The police did report officially that six different types of prescription drugs were found in the apartment, and that three of those drugs were prescribed in Europe, where Ledger was filming last week.
Ambien, however, is a sleeping tablet that to my knowledge is not prescribed in the UK - because I have some, prescribed by a US doctor, and when I told my doctor about them he asked to see the bottle because he did not know what drug this way.
Sleeping tablets are not necessarily incompatible with anti-anxiety drugs, I've been prescribed and taken both in tandem.
But he may have intentionally or unintentionally combined them to fatal effect.
However, where there is no clear evidence of suicide, and no evidence of foul play, then normally the coroner is bound by law to record a verdict of accidental death, as happened in the case of Anna Nicole Smith, although it's entirely probably she committed suicide.
Ditto Marilyn Monroe.
But with no clear evidence of this, then the death is almost always ruled accidental.