Post-mortem results on Hollywood star Heath Ledger have proved inconclusive, US authorities have said.A spokesman for the medical examiner's office said further tests are needed and could take around ten days.The Oscar-nominated actor was found dead on Tuesday in his apartment in New York.The police are treating his death as possibly drug-related and sleeping pills were found close to his body.There are also unconfirmed reports that the Brokeback Mountain actor was suffering from pneumonia at the time of his death.Ledger was discovered at 3.26pm local time by his housekeeper when she tried to wake him for his appointment to have a massage.He was found face down at the foot of his bed and prescription pills were lying nearby.In Australia, Ledger's father Kim spoke of his family's grief and said the actor's death was accidental."We, Heath's family can confirm the very tragic, untimely and accidental passing of our dearly loved son, brother and doting father of Matilda," Mr Ledger said in Perth."He was a down to earth, generous, kind-hearted, life-loving, unselfish individual who was extremely inspirational to many."Australian Prime Minister Kevin Rudd said the country had lost one of its top actors.He said: "Heath Ledger's diverse and challenging roles will be remembered as some of the great performances by an Australian actor."Fellow Australian star Nicole Kidman said it was a "terrible tragedy" and sent her condolences to Ledger's family while Mel Gibson, who played his vengeful father in The Patriot, said: "I had such great hope for him."The star was reported last year to have broken up with his longtime companion, actress Michelle Williams, who played his wife in Brokeback Mountain and was the real-life mother of the actor's young daughter, Matilda.Ms Williams is travelling back from Sweden to New York and her father said: "Tennyson got it right in the poem when he described someone as having died at a young age but burning the candles at both ends, and oh what a beautiful flame he made."Ledger is currently starring in I'm Not There, one of several actors playing a role representing Bob Dylan, and had just finished playing The Joker in the next Batman film, The Dark Knight.Ledger's other film credits included A Knight's Tale, Monster's Ball and Ten Things I Hate About You.© Independent Television News Limited 2008. All rights reserved.