FBI files obtained by Minnesota Public Radio published Monday shed new light -- and perhaps shadows -- onto the life and death of former Democratic Minnesota Sen. Paul Wellstone, who was killed when his twin-engine plane crashed in 2002.

Minnesota Public Radio (MPR) published dozens of FBI files they obtained on Monday. Their series, titled "The Wellstone Files," tracks the bureau's documentation on the onetime senator from his college days through the investigation of his death. Wellstone was killed along with his wife and six others 175 miles north of Minneapolis in the crash.

"The U.S. Department of Justice released 88 of the 125 pages in Sen. Wellstone's FBI file, and 131 of the 227 pages in his wife's file," MPR reporter Madeleine Baran wrote Monday. "All of the documents included in Sheila Wellstone's file are related to the plane crash that killed the couple and their daughter Marcia.

"The FBI did not include 76 pages related to the National Transportation Safety Board, the agency that investigated the crash," Baran added. "A request for those records is pending."

The 2002 crash has been the subject of considerable investigation, as no voice data cockpit recorder was recovered (data recorders weren't required for the small plane). The NTSB ultimately blamed the "flight crew's failure to maintain adequate airspeed, which led to an aerodynamic stall from which they did not recover," and said other elements -- such as ice -- didn't play a major role in the accident.

But the FBI files show that the bureau did investigate an alleged threat surrounding the plane's de-icing equipment. According to the files, MPR reports that "FBI agents investigated the claims of a caller from Jacksonville, Florida, who said that members of the American Trucking Association had planned to disconnect the plane's de-icers."

"The man said that Wellstone had been trying to schedule Senate hearings to expose organized crime in the trucking industry," MPR writes. "In response to the call, a Wellstone staff member asked a Labor Committee member and a legislative director 'who both indicated that they were not aware of any Senate hearing being scheduled to discuss this topic.' The rest of the document has been redacted.'"

Adds MPR's Baran:

Agents also obtained a threatening postcard sent to Wellstone's St. Paul office the day before the plane crash. The handwritten postcard said, in part, "We need to gut (sic) the word out for the snipper (sic) to go after people like you, not real Americans … This voter fraud you propose will get you dead."

An FBI agent noted that the handwriting and stamp were similar to those sent to two members of the U.S. House of Representatives who, along with Wellstone, voted against the October 2002 resolution authorizing the Iraq War.

FBI agents also interviewed a former employee of Executive Aviation, the company that employed the pilots who died in the crash. A heavily redacted report describes a conversation between an FBI agent and the former employee regarding a November 2000 incident at the company's airplane hangar.

At a closed meeting the night of the crash, the NTSB directed the investigation, with assistance from the FBI and law enforcement agencies. During the initial investigation, NTSB investigators noted several problems that the agency would later identify as key factors in the crash, including the plane's low speed, unusual sharp left turn, and the lack of any apparent problems with the plane's equipment.

FBI agents met with the lead investigator for the NTSB the following day and handed over the results of its investigation. The NTSB investigator said the agency would continue to examine the wreckage for any sign of damage to the plane, including the deicing equipment, and would interview local witnesses and investigate any previous issues with Executive Aviation.

The Duluth FBI agents and the Evidence Recovery Team left the crash site on October 28, and the FBI files do not refer to any subsequent investigations.

There were reports of icing "in the area" before and after the crash, but a meteorological expert told Minnesota Public Radio in 2003 that icing didn't appear to be "particularly severe."

There were, before and after the crash, reports of icing conditions in the area. But pilots who flew there that day reported only moderate icing at worst. Still, the pilot in command of the Wellstone flight, Capt. Richard Conry, called off the trip after his first weather briefing...

"What we look at is the different weather data that's available for the case," says aviation research meteorologist Ben Bernstein, of the National Center for Atmospheric Research, which analyzed the weather at the time of the crash at the request of the NTSB.

He declined to discuss whether icing could have brought down the plane. But Bernstein told Minnesota Public Radio that his analysis for the NTSB concluded icing was not likely a major problem at the time the plane crashed.

"There's no way for us to know for certain how severe the conditions may have been, but looking at the data we did look at, it didn't appear to be a particularly severe situation," according to Bernstein.

The NTSB ultimately blamed human error for the crash.

"During the later stages of the approach," the Board said, the pilots "failed to monitor the airplane's airspeed and allowed it to decrease to a dangerously low level (as low as about 50 knots below the company's recommended approach speed) and to remain below the recommended approach speed for about 50 seconds." The plane then stalled, and did not recover.

"The Board judged that while cloud cover might have prevented the flight crew from seeing the airport, icing did not affect the airplane's performance during the descent," the Board's press release read. "Cockpit instrument readings on course alignment and airspeed should have prompted the flight crew to execute a go-around.

"The Board did not find indications of any preexisting medical or other physical condition that might have adversely affected the crew's performance during the accident flight," it concluded. But a "review of flight crew records and interviews with co-workers, the Board said, indicated that both pilots had 'previously demonstrated serious performance deficiencies consistent with below- average flight proficiency.'"