Michelle Williams still asks why Heath Ledger died

Actress Michelle Williams says she still can't find the meaning behind Heath Ledger's death three years ago, in a rare interview talking about the late father of her child who died of a prescription overdose.

Williams, who since the actor's death in New York in January 2008, has remained mostly quiet in interviews about her former partner, told ABC news show Nightline this week she could understand why the public was curious about her grief, but still could not understand why Ledger died.

"I've found meanings around the circumstance but the actual event itself still doesn't have a ... I can't find a meaning for it," she said with tears in her eyes.

The 30 year-old actress, whose romance with Ledger blossomed on the set of Brokeback Mountain in roles for which they both received Oscar nominations, said she had found "meaning in things and people and relationships that have sprung up and friendships that have strengthened.

"I can find a lot of meaning in that, but not in why," she said.

Williams had split with Ledger when he died but was still sharing duties raising their daughter.

To deal with her grief, she said she read "The Year of Magical Thinking" by Joan Didion, the Pulitzer-Prize winning account of Didion''s year following the death of her husband.

"In a strange way, I miss that year, because all those possibilities that existed then are gone," Williams told ABC.

"It didn't seem unlikely to me that he could walk through a door or could appear from behind a bush. It was a year of very magical thinking, and in some ways I'm sad to be moving further and further away from it."

She said that for a time, she pondered the notion that one's life can change in an instant.

"I got kind of obsessed with that for a while, before and after," she said.

"A lot of things died. There is a line from a book that gave me so much comfort and it said, "When you have truly lost everything, then at least you can become rich in loss'."

Williams, who is widely tipped to be nominated for a best actress Oscar this year for her new film, Blue Valentine, said she aimed for a normal upbringing for Ledger's toddler daughter, Matilda.

"It is of more importance to me than anything else in my life," she said.