The February derailment of a coal train that dumped more than 4,000 tons of coal on the Fond du Lac Reservation was caused by a broken wheel, BNSF Railways said.

BNSF spokesperson Amy McBeth said the company determined the cause of the Feb. 16 derailment through its own investigation and reported the findings to the Federal Railroad Administration.

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The Federal Railroad Administration and the National Transportation Safety Board can choose which incidents to investigate and did not investigate the February derailment, McBeth said.

"BNSF conducts a thorough review of every incident because we want to understand what happened and why, and work to prevent it from happening in the future," McBeth said.

The BNSF train was made up of 121 loaded coal cars and three locomotives, 40 coal cars derailed at about 11:30 a.m. several miles north of Cloquet and along the St. Louis River, sending at least 4,000 tons of coal along the track and onto and into the frozen St. Louis River.

Several days after the derailment, officials from the Fond du Lac Band of Lake Superior Chippewa and nearby emergency agencies said BNSF failed to notify them of the derailment for hours and restricted access to the site.

All recoverable coal was removed by late March. On Thursday, McBeth said any coal remaining on site will be removed as weather allows.

"About 99 percent of the coal has been removed, with just a very small amount remaining," McBeth said. "As weather allows the little coal left will be removed and we'll be seeding the area to restore it to the pre-incident condition."

A Fond du Lac spokesperson did not immediately respond to the News Tribune on Thursday.

The coal was heading to Minnesota Power's Boswell Energy Center, the company's only operating coal-fired power plant, from its source in the Powder River Basin in Wyoming.