ST. PAUL — Minnesota Public Radio reports the FBI investigated several threats against the late Sen. Paul Wellstone but never prosecuted anyone for them.

MPR is citing FBI files it obtained through a Freedom of Information Act request.

The files show the threats started shortly after the Minnesota Democrat’s arrival in Washington in January 1991, after he beat incumbent Republican Sen. Rudy Boschwitz. The outspoken liberal took office amid a tense debate over the first gulf war, which he opposed.

MPR reported Monday that the files also show the FBI first took interest in Wellstone in 1970 when he was arrested during a protest against the Vietnam War. And they show the FBI investigated the 2002 plane crash that killed Wellstone but found no indication of criminal activity.