Massachusetts regulators on Monday ordered National Grid plc to halt all nonessential work in the state after excess natural gas was “inadvertently” introduced into pipelines, raising concerns of a repeat of last month’s Merrimack Valley explosions.

Following the incident, National Grid shut off gas to about 300 homes in Woburn, which is about 11 miles north of Boston. National Grid supplies power and gas for more than 20 million people in New York, Massachusetts and Rhode Island and is the largest gas distributor in the Northeast.

The Department of Public Utilities (DPU) imposed a moratorium on National Grid’s Massachusetts operations following the incident “on all work, except for emergency and compliance work, across the company’s entire service territory, pending the results of DPU’s review of National Grid’s safety practices,” said spokesman Peter S. Lorenz of the state’s Executive Office of Energy and Environmental Affairs.

“In addition, the department is requiring National Grid to have an inspector on location for all work that could lead to abnormal pressurization until this review is complete.”

DPU late last month said it would hire an independent evaluator to examine the safety of the state’s entire gas distribution system following a series of fatal explosions and fires on Sept. 13 on a system overseen by NiSource Inc. unit Columbia Gas of Massachusetts.

Emergency responders swarmed the Merrimack Valley communities of Lawrence, Andover and North Andover after a series of gas explosions and fires killed one man, injured about 25 people and destroyed or damaged dozens of homes. The National Transportation Safety Board is investigating.

According to National Grid, a gas technician was conducting routine maintenance in Woburn on a regulator station about 11:30 a.m. ET on Monday, when he “inadvertently introduced excess gas.” The company around 2:30 p.m. began to notify residents that gas would be shut off.

“The crew quickly recognized the error, and within minutes, reduced the system to normal operating pressures,” National Grid stated. “The area is safe, and National Grid has the situation under control.”

Technicians are assessing the system, but there was no official timeline as to when gas service would be restored.

A National Grid spokeswoman said the work is being done by “management employees with decades of experience, who would ordinarily be overseeing the work of our union employees.”

About 1,000 members of the United Steelworkers union have been locked out of work since their contract ended in June. The union complained about not being involved.

“They’re endangering public safety by having inexperienced people deal with gas, something potentially very dangerous,” United Steelworkers’ Neil Crowley said of the National Grid assessments. “They’re supervising things they’ve never worked directly with.”

During a press conference on Monday to discuss the incident, Woburn Fire Chief Stephen Adgate was asked how concerned he was when he learned about the National Grid’s incident.

“I started to get chest pains; does that tell you anything?” he reportedly said.

Meanwhile, Massachusetts’ two Democratic Sens. Edward J. Markey and Elizabeth Warren charged that Columbia Gas of Massachusetts was “woefully unprepared” for the Merrimack Valley incident and “did not properly contemplate the possibility” that a disaster of that magnitude could occur.

In a letter outlining their concerns, which was sent last week to Columbia Gas of Massachusetts and NiSource officials, the senators said they reviewed updated operating policies put in place by Columbia Gas five days after the tragedy. They questioned why “common sense safety measures were not in place, such as knowing the exact location of control lines and ensuring the qualified company personnel are onsite during the duration of any work.

“The omission of these sorts of safety measures from Columbia Gas’ operating procedures prior to this disaster is alarming and unacceptable,” Markey and Warren wrote. “It raises serious questions as to why these policies were not previously in place for Columbia Gas’ systems and whether that failure was the result of negligence, cost considerations, or incompetence.”