The FBI investigated whether Anna Nicole Smith plotted to kill her tycoon husband's son as they battled for his father's fortune, newly released files show, but the former Playboy Playmate who died in 2007 was never prosecuted.

Smith's FBI records, obtained exclusively by The Associated Press, say the agency investigated Smith in 2000 and 2001 in a murder-for-hire plot targeting E Pierce Marshall, who was at the center of a long legal fight to keep the starlet, model and one-time stripper from collecting his father's oil wealth, valued in the hundreds of millions. The younger Marshall died three years ago of natural causes.

The documents released under the Freedom of Information Act depict an investigation going on as the fight raged over J Howard Marshall II's estate.

Vast sections of the 100 pages of released materials - a fraction ofSmith's full FBI file - are whited out, and no evidence of her involvement in such a plot is detailed.

There is no indication how authorities became aware of any alleged scheme, but agents interviewed Smith on July 3, 2000. Told why she was being questioned, "Smith began crying and denied ever making such plans," a report said.

Smith told agents that killing Pierce Marshall would not have made sense because her ex-husband's money would still be tied up in a trust, and because she believed the court battle over the fortune was nearly over, the report states.

She told agents she believed the story about the plot was made up by a former lover angry that she broke off their relationship, according to the files.

"Smith adamantly denied ever contemplating such a crime," an agent wrote, and prosecutors eventually agreed the case could not go forward.

An April 26, 2001 letter to the FBI from Sally Meloch, an assistant US attorney, said she reviewed the reports but "determined that there is insufficient evidence to establish that there was a murder-for-hire plot by Ms Smith to kill Pierce Marshall."

Reached at her Los Angeles office on Tuesday, Meloch didn't recall the case, but said, "Any investigations that we didn't proceed with, we couldn't comment on anyway."

Ron Rale, Smith's personal attorney during her life and the executor of her estate, said whatever allegations were made were absolutely false: "Unequivocally, there was never a murder-for-hire plot that Anna was involved in. Absolutely false. Never happened. End of story."

An attorney for the Marshall estate, including for the younger Marshall's widow, said he couldn't comment.

A spokesman for the family, David Margulies, said the family was aware of the FBI investigation but didn't wish to comment further.

But Rusty Hardin, a Houston attorney and longtime Marshall family friend, laughed upon hearing of the alleged plot.

"I don't think I ever saw any credible evidence," he said. "I do not remember ever being concerned about whether she was trying to have him killed."

Smith was 26 when she wed the 89-year-old Marshall, owner of Great Northern Oil Co, whose wealth was estimated by Forbes to be $550 million in 1992. They met while she was a topless dancer at a Texas strip club.

He died of natural causes in 1995, little more than a year after they wed. His son died in 2006 at age 67 of an infection and Smith died of a drug overdose a year later at age 39 after collapsing in her South Florida hotel room.

The FBI files show a .357-caliber Smith and Wesson revolver was confiscated from Smith's home, along with a 3 -inch stainless steel knife and, for reasons that were not explained, a black-and-orange hat described as "Dr Seuss." All three objects were returned to her about seven months later.

The dispute between Smith and the Marshall estate has bounced around courts for years.

The Supreme Court ruled unanimously in 2006 that Smith could pursue her late husband's fortune.

The money became a factor after Smith's death, too, with Stern, her mother, and another boyfriend all fighting over an estate that ultimately will go to her daughter, who is now 3.

Smith's lawyer and companion Howard K Stern and two doctors, Dr Sandeep Kapoor and Dr Khristine Eroshevich, are charged in California with helping the model obtain drugs that ultimately killed her. All have pleaded not guilty.