The pressure in natural gas pipelines prior to a series of explosions and fires in Massachusetts last week was 12 times higher than it should have been.

The information was in a letter from the state's U.S. senators to the heads of Columbia Gas, the company that serves the communities of Lawrence, Andover and North Andover, and NiSource, the parent company of Columbia Gas.

Democratic Sens. Elizabeth Warren and Edward Markey sent the letter Monday seeking answers to questions about the explosions.

The letter says that according to the Pipeline and Hazardous Materials Safety Administration, "the pressure in the system should have been around 0.5 pounds per square inch, but readings in the area reached at least 6 PSI."

The explosions and fires killed one person and injured more than two dozen.

Columbia Gas in statement Friday morning said it is "working with the appropriate authorities to investigate this incident in order to understand its cause."

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