Last September, 18-year-old Leonel Rondon died after a series of fires and gas explosions rocked three towns in Massachusetts' Merrimack Valley.
After over a year of investigations, the National Safety Transportation Board says it finally knows what caused the destruction -- inadequate management and poor oversight that led to a cast iron pipe being improperly abandoned.
In the town of Lawrence, the fires and explosions brought a chimney down on the car Rondon was sitting in, and he later died at Massachusetts General Hospital.
The destruction resulted from over-pressurization of the area's natural gas distribution system. At least 22 other people were transported to area hospitals, 50,000 residents were evacuated, and at least 131 structures were damaged. More than 180 fire departments and over 140 law enforcement agencies -- with over 1,000 officers -- responded to the incident.
"The NTSB determines that the probable cause ... was Columbia Gas of Massachusetts' weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main," NTSB's Managing Director Sharon Bryson said Tuesday. "Contributing to the accident was a low-pressure natural gas distribution system designed and operated without adequate over-pressure protection."
The probable cause was unanimously adopted on Tuesday by NTSB Chairman Robert Sumwalt, Vice Chairman Bruce Landsberg, and Board Member Jennifer Homendy after presentations by NTSB investigators at a board meeting in Washington, DC.
NiSource, the parent company for Columbia Gas, praised the NTSB investigation. "Our own understanding of the events generally aligns with that of the NTSB," the company said in a statement. "We welcome today's action by the NTSB because it will help us, our industry partners, the public, and others learn from this tragedy. As we've said since that tragic day, we take responsibility for what happened."
The fires and gas explosions on September 13, 2018 affected residents across Lawrence, North Andover, and Andover, Massachusetts. The damage was so significant that Massachusetts Gov. Charlie Baker declared a state of emergency the following day. Hundreds of natural gas technicians arrived in the towns in the following days to restore gas service safely before electricity could be turned on, state officials said.
While many residents were allowed to return to their homes within three days, a multitude of homes did not have heat, gas, and hot water services restored to their homes until December 2018.
On Tuesday, the NTSB also unanimously adopted 14 investigative findings that faulted Columbia Gas of Massachusetts in several areas, including: inadequate planning, documentation, and record-keeping processes, an insufficient review process, and failure to follow their own emergency response procedures.
Additionally, they stressed the need for better emergency planning responses across different towns, citing the poor communication among separate fire departments responding to the fires.
Five new recommendations were also made to 31 states, NiSource, and the Pipeline and Hazardous Materials Safety Administration, urging them to review protocols and training for large scale emergency events, raise their standards for infrastructure project approval, and issue alerts to other low-pressure natural gas distribution system operators of the possibility of a failure of over-pressure protection.
CNN's Ray Sanchez and Nicole Chavez contributed to this report.