Prosecutors never charged Smith, who died in 2007, because of insufficient amount of evidence. The investigation took place in 2000 and 2001, when she was battling her former son-in-law, E. Pierce Marshall, over the Marshall family fortune.
Pierce Marshall died from an infection in 2006 at age 67.
FBI records obtained by the AP contained allegations that Smith was a part of a murder-for-hire plot against the younger Marshall but no further details of her involvement were released. The records also showed that the FBI confiscated a .357 Smith and Wesson revolver and a 3.5-inch knife from Smith during the investigation. Both items were returned seven months later.
An agent wrote: "Smith adamantly denied ever contemplating such a crime."
Smith married Marshall's father, oil tycoon J. Howard Marshall II, in 1994 when she was 26 and he was 89. After a year of marriage, he died of natural causes.
In a recorded June 27, 2000, interview, Pierce Marshall told the FBI that Smith rarely spent time with his father once they were married. He also said his father complained that Smith asked for $50,000 to $60,000 twice a week, according to the AP.
Howard Marshall's death sent Smith and Pierce Marshall into a bitter legal dispute that continues over his estate.
The Supreme Court ruled unanimously that Smith was allowed to pursue Howard Marshall's estate in 2006. The ruling, however, overturned an appellate court decision, which continues to be fought in California. Since Smith's death in 2007, whatever money she may receive in the future will go to her 3-year-old daughter.