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.”