National Transportation and Safety Board Chairman Robert Sumwalt says his organization is here not only to investigate what caused dozens of gas-related fires in the Merrimack Valley on Thursday, but to see how the disaster was handled by the company who owns and maintains the pipes that caused them.

“Our mission is to find out what happened, so it doesn’t happen again,” said Sumwalt.

At a press conference on Saturday, Sumwalt indicated that there was a “pressure increase” noted in a control room in Columbus, Ohio, that monitors the pipeline that supplies gas to the affected homes and businesses in Andover, North Andover and Lawrence.

Sumwalt said the NTSB will be pulling the documents from the control room and speaking with the controller who noted the increase to find out when the increase was logged and the extent of it. He said he expects this information to be available tomorrow.

Sumwalt also laid out exactly what NTSB’s investigation would entail. He said this investigation will determine the cause of what he deemed an "accident" and not when people can return home and when their electricity or gas would be restored.

The team will first collect “perishable” evidence and document damage to property through witness testimony and record reviews, he said. The NTSB will review complaints of “gas smells” over the last three weeks, and investigators will also look into whether that number increased over time.

The NTSB investigation will take a deep dive into Columbia Gas and their policies, including prepared safety plans with first responders in the event of a gas event and what their company “culture” was like when it came to safety, Sumwalt added.

NTSB officials will also test safety mechanisms along the pipeline that provides gas to parts of Andover, North Andover and Lawrence. According to Sumwalt, there are 14 gas pressure regulators in the region, which is a “normal” amount. The investigation will also look into whether that number should be higher.

Sumwalt said he anticipates the onsite portion of the NTSB investigation to last between one and two weeks.