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    Pressure inside Columbia Gas pipes was 12 times higher than normal

    The pressure inside pipes carrying natural gas through three Merrimack Valley communities was recorded to be at least 12 times higher than it should have been Thursday when dozens of gas-fueled explosions and fires erupted, U.S. Senators Edward J. Markey and Elizabeth Warren said Monday, citing briefings from federal regulators.

    The excessive amount of pressure was revealed in letters the two lawmakers sent on Monday demanding answers about the incident from the heads of Columbia Gas of Massachusetts, its parent company NiSource, and federal regulators at the Pipeline and Hazardous Materials Safety Administration, giving both a deadline of Wednesday to answer.

    “We write to request that you provide us with information in order to help the American people understand why this terrible disaster occurred, whether the company was sufficiently prepared to respond to an incident of this magnitude, and how we can prevent any similar tragedy in the future,” the letter to company executives said.

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    The letters ask more than two dozen questions, many focused on learning what exactly unfolded on Thursday, what actions the company took, and how long it took to respond.

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    The senators noted that a federal regulator, the Pipeline and Hazardous Materials Safety Administration (PHMSA), reported that the pressure in the Columbia Gas system “should have been around 0.5 pounds per square inch (PSI), but readings in the area reached at least 6 PSI.”

    Excessive levels of pressure within gas pipes can lead to leaks and explosions. Mel Ydreos, spokesman for the International Gas Union, said overpressurization in the system forces gas towards homes, where there’s less resistance. There the flammable gas can leak, causing it to buildup and creating the potential for fires and explosions.

    “Any over pressurization in a low-pressure system is not good,” said Ydreos. “I would be concerned over any level,” of excess pressure.

    The politicians requested more details on the work Columbia Gas was doing to upgrade gas mains in the Lawrence area at the time, and also requested copies of the company’s emergency response plans, and changes it made in the wake of previous failures.

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    A Lawrence teen died, at least 25 people were injured, and 80 buildings were damaged in the fires and explosions in Andover, Lawrence, and North Andover on Thursday.

    Thousands had their power cut and gas service cut and were ordered to evacuate. Governor Charlie Baker declared a state of emergency and ordered Eversource Energy to take over the utility industry’s response. Residents have been allowed to return to their homes and electricity has been restored, but gas may not be available to some businesses and residences for months because Columbia Gas said it will replaced 48 miles of pipeline.

    On Monday, Baker and other officials announced the formation of a charitable fund to funnel financial aid to residents of the three communities.

    Warren and Markey have already called for a congressional hearing on the disaster, and, along with U.S. Representatives Seth Moulton and Niki Tsongas, sent a sharply worded letter to the head of Columbia Gas last week criticizing the company’s “woefully inadequate response.”

    Columbia Gas and NiSource officials did not immediately comment Monday.

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    Officials at the federal pipeline agency said they have received the letter and noted the agency is coordinating with Massachusetts officials and the National Transportation Safety Board, which is leading the investigation into the catastrophe.

    Milton J. Valencia and Danny McDonald of the Globe staff contributed to this report. Matt Rocheleau can be reached at matthew.rocheleau@globe.com. Follow him on Twitter @mrochele