Federal investigators probing the deadly natural gas explosions that rocked three northern Massachusetts towns last month are blaming faulty work orders given to the work crews by the local utility.
The investigators say Columbia Gas failed to instruct the workers to deactivate pressure sensors when taking an old cast-iron gas main out of service.
When those sensors detected a drop in gas pressure in the decommissioned main on Sept. 13, the system automatically increased the pressure in the new, plastic gas main.
Within minutes, houses across Lawrence, Andover and North Andover began to explode from a buildup of natural gas.
One man was killed when the chimney of an exploding house fell on top of his car. Twenty-one people – including two firefighters – were injured.
The preliminary findings by the National Transportation Safety Board do not end the investigation into the incident.