Actress Mary-Kate Olsen has refused to answer questions from federal authorities investigating the accidental overdose that killed actor Heath Ledger until she receives immunity from prosecution, The New York Post reported.

The former child-star, famous for playing Michelle Tanner on "Full House," was a close friend of Ledger's and the first person called after her masseuse discovered Ledger's body in his Soho apartment in January, The Post said.

But federal officials could force the actress to tell them details on the "Dark Knight" star's drug use and events leading up to his death if they obtain a grand-jury subpoena.

One source told The Post that "everyone has been very eager to help, saying what a great guy Heath was, everyone except Mary-Kate, who has refused to speak."

When Olsen got the call that Ledger — whose performance as the Joker in "The Dark Knight" has earned rave reviews — was dead, she reportedly failed to call authorities, and instead telephoned her body guards. According to The Post, they arrived just as paramedics got to the scene.

Ledger's death was ruled an accidental drug overdose by medical examiners, caused by a combination of prescription drugs and the pain reliever OxyContin. According to The Post, the latter was likely received illegally.

Click here to read the full report from The New York Post.