Tragic Playboy Playmate Anna Nicole Smith was investigated by the FBI over a plot to kill her tycoon husband’s son, it has emerged.

Newly-released files show the famously curvy blonde, who was battling E Pierce Marshall over his late father’s oil fortune, was probed by investigators in 2000 and 2001 over an apparent murder-for-hire plot.

According to the FBI files, a revolver was recovered from Smith’s home during the investigation, as well as a three-and-a-half-inch knife.

Ultimately, prosecutors decided there was not enough evidence to charge Smith, who died in 2007 from a drugs overdose. E Pierce Marshall has also since died.

During an interview with agents on July 3, 2000, “Smith began crying and denied ever making such plans,” a report said.

“Smith adamantly denied ever contemplating such a crime,” an agent wrote.

Smith’s estate attorney has expressed surprise at the allegations, while representatives for Marshall’s estate would not comment.