NTSB investigators concluded Norfolk Southern and its contractors erred when they vented or burned the contents of five derailed tank cars that contained vinyl chloride three days after the derailment occurred.

Venting and burning the contents spread hazardous chemicals across 16 states and exposed about a third of the nation’s population to the chemicals.

The five tank cars were among 38 that derailed when a wheel bearing caught fire on a covered hopper rail car but did not trigger any alarms designed to detect such problems.

NTSB member Jennifer Homendy accused Norfolk Southern officials of trying to submit their own report four times instead of cooperating with the NTSB investigation into the derailment.

“Parties are not permitted to manufacture their own evidence,” Homendy said.

She said Norfolk Southern officials delayed sharing requested information with the NTSB and denied the existence of some information.

Homendy said she twice threatened the railroad with subpoenas to obtain the requested information for the investigation.

She called the actions of railroad officials “unprecedented and reprehensible” and said an unnamed railroad executive recently ended a meeting by threatening her and her staff. She did not elaborate on the alleged threat.

The NTSB is scheduled to release a summary of its report on the derailment as soon as Tuesday evening.

The NTSB also approved about two dozen recommendations that are not legally binding.

The recommendations include revising guidelines for venting and burning rail car contents, implementing new policies to improve communications, expanding the use of in-cab audio and video recordings, and improving bearing-defect detection systems on railroads.

The NTSB also wants Ohio officials to mandate improved training for volunteer firefighters and national firefighter groups.