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, according to a letter from the state's U.S. senators to executives of the utility in charge of the pipelines. Democratic U.S. Sens. Elizabeth Warren and Edward Markey sent the letter Monday seeking answers about the explosions from 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.