A train that partly derailed and exploded north of downtown was traveling below the recommended speed before the crash, and its operators noticed no problems with the track as they approached the curve where it happened, a National Transportation Safety Board representative said.

The NTSB is looking into what caused the Wednesday derailment on Norfolk Southern Corp. tracks north of downtown Columbus, which led to spectacular explosions and the burning of three tank cars, each carrying 30,000 gallons of ethanol.

Authorities said they decided to let the tankers burn for a while and then spray chemical foam to prevent the blaze from re-igniting.

The smoldering fire extinguished early Thursday morning, and Norfolk Southern has begun moving cars away from the scene, a company spokesman said.

Investigators are reconstructing the rails in hopes of spotting any problems. The full investigation could take a year.