Findings of the NTSB investigation:1. Greater first responder training and awareness about hydrogen tube trailer modules transported within their jurisdiction could have helped the first responders to initially respond to this incident more effectively, efficiently, and safely. 2. Although there is ample generic hazard information for compressed hydrogen and vehicle fuel systems, the available guidance lacks critical hazard recognition and firefighting information specific to fuel cell electric vehicle fuelling infrastructure and containers currently used for the bulk transportation of compressed hydrogen. 3. The generic guidance for flammable gases contained in the US Emergency Response Guidebook, Guide 115, does not adequately describe unique hazards associated with compressed hydrogen or best response actions for public safety personnel. 4. None of the incident tube trailer cylinders lost mechanical integrity, and they did not contribute to the cause or perpetuation of the incident. 5. The hydrogen release most likely initiated because a pressure relief device with the wrong pressure rating was installed on cylinder No. 14 and actuated under normal working pressure loads. 6. The hydrogen ignited during the forced separation of the cylinder No. 14 pressure relief device assembly from the cylinder. 7. Although the other cylinder pressure relief devices in the tube trailer module were actuated because of fire exposure, the cylinder No. 14 pressure relief device actuated before the fire began. 8. Had the requalification personnel inspected each vent line compression fitting for tightness, the propagation of the fire to the adjacent hydrogen cylinders may have been avoided. 9. The vent line compression fittings that disassembled during venting were not properly installed at the time the tube trailer module was fabricated and should have been noticed and repaired during the requalification inspection. 10. The lack of a Pipeline and Hazardous Materials Safety Administration special permit and regulatory requirements for verifying that pressure relief devices used on cylinders actuate at the correct pressure and venting equipment has been properly assembled and secured increases the risk of uncontrolled fires on flammable gas tube trailers.11. The lack of vent system design requirements that consider factors such as the force of venting gas and construction material may leave vent systems for cylinders and tube trailers vulnerable to unexpected failure during an incident. 12. The lack of noticeable marking or other visual indicators, unique design features, and unique model numbering to readily identify pressure relief devices intended for different applications, made them easy to confuse and did not avert the requalification technician from installing four wrong pressure relief devices on the incident tube trailer module.