NEW YORK, US - US actress Mary-Kate Olsen will not speak to federal investigators about actor Heath Ledger's death unless she is granted immunity from prosecution, a law enforcement source told Reuters on Monday.

Olsen, 22, was a friend of Ledger and the first person called by his masseuse, who found the 28-year-old Australian dead in his New York apartment in January from an accidental prescription drug overdose.

Olsen, best known for her role in TV series Full House, summoned private security guards, who arrived at Ledger's apartment in Manhattan's SoHo district at the same time as emergency services workers.

The US Drug Enforcement Administration is investigating the source of Ledger's drugs and medicines. 'We have asked to interview her (twice) and it has not happened,' the law enforcement source said.

The source said that investigators are in negotiations with Olsen's lawyers and the prosecutor's office.

Olsen lawyer Michael Miller said that, at her request, he had provided investigators with relevant information, including a chronology of the events surrounding Ledger's death.

'Mary-Kate Olsen had nothing whatsoever to do with the drugs found in Heath Ledger's home or his body, and she does not know where he obtained them,' Miller said in a statement.

Ledger was nominated for an Oscar for his role as a gay cowboy in 2005's Brokeback Mountain. His final role as the Joker in the Batman sequel The Dark Knight, released last month, is being critically hailed with Internet buzz touting him as an Oscar candidate in 2009.

Ledger has a two-year-old daughter, Matilda, with his Brokeback Mountain co-star Michelle Williams. -- REUTERS