BOSTON — Lawmakers investigating the Registry of Motor Vehicles scandal are growing increasingly frustrated with Governor Charlie Baker's transportation department, warning in a Tuesday letter that they are "losing confidence in the Administration's willingness to cooperate fully" and hinting they may soon seek subpoena power to acquire relevant records.

In a terse letter to Transportation Secretary Stephanie Pollack, Rep. William Straus and Sen. Joseph Boncore expressed "overall frustration and deep concern" that their requests for documents, particularly internal communications among RMV employees, have not been fully addressed.

No additional documents or files have been provided since July 29, they said, the day before the Transportation Committee grilled current and former RMV officials as well as Pollack for more than seven hours about how the Registry allowed a backlog of out-of-state violation notices to build up for years despite multiple warnings.

"The Committee's intention to conduct a thorough and complete oversight of the RMV has been made clear several times over the last few weeks; the failure to complete our pending requests with additional production from MassDOT lacks justification," Straus and Boncore, who both chair the committee, wrote. "This is the last time the Committee will respectfully request these documents."

Straus told the News Service in a follow-up interview that, although he had hoped to handle the investigation with "cooperation and openness," the lack of response to their document requests may push the committee — which does not currently have subpoena power — into legal action.

"There does come a point, and we'll see what the next several days bring, where the only further option is to seek permission from the full House and the full Senate for the grant of subpoena authority," Straus said. "It's not how we want to do this. But the importance of the information we're seeking would require that step if we don't begin seeing the full compliance that we think had been indicated would occur."

A spokeswoman for MassDOT said the administration has cooperated so far with the committee's requests for documents, and will continue to do so as it works simultaneously to rectify the situation at the RMV.

"MassDOT has turned over hundreds of documents to the committee during the last several weeks and the Department is gathering additional documents to comply with the committee's request while working to correct deficiencies at the Registry of Motor Vehicles," spokeswoman Jacque Goddard said in a statement to the News Service.

Straus and Boncore raised particular concern about a Boston Herald story from last week about an employee of the Merit Rating Board raising concerns about staffing levels in a 2016 email, shortly before the department became responsible for taking action against Massachusetts drivers flagged by other states.

That email was never provided to the committee despite a request for all written communications sent by RMV employees regarding out-of-state notifications or MRB processing traffic offenses, Straus and Boncore said.

"It raises a question: if there are one or two things that are not provided, are there more?" Straus told the News Service. "I just can't say right now."

The chairs had previously hinted at seeking subpoenas after their first attempt at holding an oversight hearing, when more than half of the witnesses they sought did not show up.

But all witnesses were on hand for the second hearing date last week, when members asked pointed questions that revealed systemic failures within the RMV to act on warnings from other states.

Former Registrar Erin Deveney, who resigned in June after a West Springfield man who should have had his commercial license suspended based on a Connecticut arrest allegedly killed seven in a New Hampshire crash, told lawmakers that there had been no formal process to handle the notifications until 2016.

Even after that, RMV employees testified, the registry often did not have sufficient staffing to handle the task and lacked a clear sense of whose responsibility it was, and tens of thousands of notices built up untouched. Several warnings from both inside and outside the RMV flagged the issue, but it was never sufficiently addressed.

Deveney and other RMV managers drafted a memorandum to administration officials in October 2016 about a backlog of warnings, but Pollack told the committee the document was never actually sent and that she only learned of the issue following the June crash — the same timeline of awareness offered by Baker.

Straus said the committee is likely to hold another hearing to get testimony from several additional witnesses, including a representative from the company that designed the RMV's records system and from audit firm Grant Thornton that is investigating the failures. Both attended last week's hearing, but were not called on to testify.

Additional witnesses may be sought, Straus said, but he did not say whether the next hearing would come during the traditional August recess or later in the fall.

"That depends on how long it takes to get the documents and records we've requested," he said.

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