Federal investigators have concluded the Georgia Department of Transportation is partly to blame for the fire that destroyed part of I-85 in Atlanta and crippled the region's traffic for weeks.

Wednesday, the National Transportation Safety Board released a report that found the department's decision to stockpile construction materials under I-85 contributed to the inferno that destroyed a section of the highway in Buckhead.

The report concluded the “large amount of combustible material being stored underneath this section of the I-85 bridge increased the fire risk to the bridge.” It cited GDOT’s “failure to assess the increased fire risk due to the presence of these combustible materials.”

This image from Google Maps Street View shows the spools of high-density polyethylene conduit in September 2014, stored under the portion of I-85 that collapsed last week. The conduit is blamed as causing the fire to burn hot enough to collapse the bridge.

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The NTSB also issued a safety alert to transportation officials across the country.

“Although catastrophic fires fueled by materials stored underneath bridges are relatively rare events, the loss of this structure demonstrates what can happen if bridge owners are not vigilant about monitoring and controlling such materials,” the NSTB warned.

GDOT issued a statement welcoming investigators’ input and saying it has already taken steps to ensure nothing similar happens again.

“Last year, GDOT made changes in its storage practices within hours after the bridge collapse, and we remain committed to building on the changes we have already implemented, which specify no storage of flammable or combustible materials under bridges,” the statement said.

Police have said that on March 30, 2017, a homeless man, Basil Eleby, was using drugs when he set fire to a chair on top of a shopping cart. The cart melted, and the blaze quickly spread to high-density plastic conduit GDOT had stored under the bridge for years.

A aerial view of the portion of I-85 that collapsed during a massive fire on March 30, 2017. BOB ANDRES /BANDRES@AJC.COM

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The fire destroyed 350 feet of I-85 in both directions. It took GDOT and a contractor six weeks to rebuild the structure.

In the wake of the incident, questions swirled about who was to blame.

GDOT has said the main cause of the fire was arson. In December, Eleby’s case was transferred to Fulton County’s Behavioral Health Treatment Court. If he completes a mental health court program, the charges against him will be dropped.

Others insisted GDOT shared some of the blame for the fire because it was responsible for the construction material that fueled the blaze.

At the time, GDOT said it was common practice to store such materials under highway bridges. But The Atlanta Journal-Constitution found several states that prohibit the practice.

GDOT also said the materials were secured behind a locked gate, but the newspaper found evidence that the gate was sometimes left open.

The NTSB and the State Fire Marshal launched investigations. Last year, a state report suggested GDOT should reconsider storing construction materials under highways. Wednesday's NTSB report took that message to a national audience.

The NTSB investigates significant highway accidents to determine the probable causes and issues safety recommendations to prevent other accidents.

Its safety alert said agencies responsible for bridges should evaluate materials stored under them. It also suggests they work with local law enforcement to improve bridge surveillance and lock or otherwise protect storage areas to restrict entry.

In Wednesday’s statement, GDOT said it cooperated in the preparation of the NTSB alert and is pleased with the “valuable guidance.”

“We are hopeful that these recommendations from the NTSB will be instrumental for other relevant agencies and departments of transportation across the country to prevent instances like this from happening elsewhere,” the statement said.


NTSB finding:

“… The probable cause of the fire and subsequent collapse of the span 30 NB bridge structure on Interstate 85 north, in Atlanta, Georgia, was excessive heat from the ignition of 76 reels of high-density polyethylene conduit and nine racks of fiberglass conduit stored beneath the overpass. Contributing to the bridge collapse was the decision of the Georgia Department of Transportation to store construction materials beneath the bridge and its failure to assess the increased fire risk due to the presence of these combustible materials.”