Last month, federal overseers gave Massachusetts utility regulators top marks on their ability to perform safety inspections on gas lines across all categories except one: the state did not have enough active inspectors in the field just three weeks before a series of gas explosions and fires rocked the Merrimack Valley.

"Having a full complement of inspection staff is essential toward meeting the requirements of the pipeline safety grant program," Zach Barrett, the U.S. Department of Transportation's director of state programs, wrote to Massachusetts Department of Public Utilities commissioner Angela O'Connor on September 17.

The report expressed concern that retirement and attrition through 2017 and 2018 could delay the implementation of inspection plans, and noted that the DPU "is taking steps to fill vacant inspector positions."

The agency has employed an average of around 10 engineers, not all of them certified inspectors, since 2012: eight in 2018, ten in 2017, 12.5 in 2016, 11.5 in 2015, 12 in 2014, 8 in 2013 and 8 in 2012, according to the state's Executive Officer of Energy and Environmental Affairs, which oversees the DPU.

When the federal officials visited this summer, six of the eight inspectors were not available to conduct inspections.

"At the time of this evaluation only two inspectors are available to perform inspections. The program is taking steps to fill vacant positions," the report states.

According to EEA, one inspector is on medical leave, another is assigned to office work for medical reasons, two directors are not active field inspectors and two new hires had not completed training at the time of the federal evaluation. Two remained to carry out the work of the inspection program.

Even with the short staff, the report found that inspections by DPU employees has been trending up every year since 2013. According to EEA, the DPU completed 1,066 inspections in 2017, 880 in 2016, 822 in 2015, 635 in 2014, 716 in 2013 and 795 in 2012.

"Staff training will be an area of emphasis due to employee turnover over the past year. New inspectors will need to attend and successfully complete all mandatory training for gas inspectors," Barrett wrote in the letter to O'Connor

O'Connor received another letter Monday, this time from Jay Gonzalez, the Democratic nominee for governor, who says Baker's oversight of the DPU has left the state's gas infrastructure vulnerable.

"It sounds like they've been all over the place in terms of number of inspectors, how many inspectors are actually doing inspections, how many are certified to do inspections, how long this has persisted," Gonzalez told WGBH News after hand-delivering the letter.

"If the federal government is saying they weren't adequate then that raises real questions about the Baker administration's credibility about whether they are or not," Gonzalez said.

Gonzalez is trying to deflate the idea that Baker is a master manager and is questioning whether the DPU had enough inspectors on staff to keep up with safety requirements.

It could be some time before anyone affected by the Merrimack explosions finds out if the state's oversight was at all to blame for the fires that left one dead, dozens injured and scores more without heat as summer turns to fall.

A review of the incident is being carried out by the National Transportation Safety Board and is still in its earliest stages. NTSB spokesman Keith Holloway told WGBH News the agency generally issues a preliminary report a few weeks after it finishes collecting evidence in the field, but that the first report will not discuss the incident's cause.

A full report on what caused the Merrimack gas explosions, the culpability of operator Columbia Gas and the state's role could have to wait until 2020.

"NTSB investigations are comprehensive and take considerable time to complete. Major investigations, such as this one, currently take between 12 and 24 months to complete," Holloway wrote.

What Democrats like Gonzalez see as a half-empty staffing situation at the DPU, Baker sees as at least half-full and has consistently defended the performance of the agency.

"The report gives the DPU a 'perfect score' for its operations, and the agency is performing significantly more inspections compared to 2013," Baker-Polito campaign spokesman Terry MacCormack told WGBH News, adding that the governor is focused on the recovery of the Greater Lawrence area instead of "scoring political points."

The DPU scored 112 out of 115 possible points when the DOT evaluated its operation in July and August 2018.

"The score indicates that the State of Massachusetts has provided your agency sufficient statutes, regulations, people and training to conduct your Gas Pipeline Safety Program. Congratulations on a perfect Progress Report score," Barrett wrote.

The missing points were deducted for a clerical error and because none of the inspectors had completed Integrity Management Program training, a required program. The report notes that a IMP-certified inspector has recently retired and an active inspector had been tasked with completing the training to take his place. The third demerit resulted from poor verification of inspectors' implementation of that training when monitoring utility operators.

Editor's Note: This story has been edit to reflect that natural gas inspectors employed by the state are engineers.