Mary-Kate Olsen has beefed up her security after receiving hate mail and death threats from angry Heath Ledger fans who blame the actress for the recent death of her former flame, NW reports.

"A lot of people who knew Heath and Mary-Kate in his last months know that they were partying together constantly," says an insider. "It wasn't hard to connect the dots and run a line between Heath hooking up with Mary-Kate and his sad decline."

"One crazed fan sent her threatening letters. Another made menacing phone calls," says a friend. "Mary-Kate's gotten more reclusive, paranoid and afraid to walk the streets than ever."