BOSTON — Massachusetts legislators probing the state’s Registry of Motor Vehicles in the aftermath of a crash that killed seven motorcyclists are growing impatient with state officials’ response to document requests and “losing confidence” in the willingness of Gov. Charlie Baker’s administration to cooperate with the investigation, according to a new letter.

“We are writing to express the Committee’s serious, overall frustration and deep concern with the level of cooperation in the document process thus far,” the House and Senate chairs of the Legislature’s Transportation Committee wrote in the letter delivered Tuesday to state Secretary of Transportation Stephanie Pollack.

The panel is probing the Registry’s failure to act on notices it received that Volodymyr Zhukovskyy, the commercial truck driver from Massachusetts involved in the deadly June 21 crash in New Hampshire, had been arrested weeks earlier on a drunken driving charge in Connecticut. That information, Massachusetts officials have acknowledged, could have triggered suspension of Zhukovskyy’s license before the New Hampshire crash.

In their letter, Democratic Rep. William Straus, of Mattapoisett, and Democratic Sen. Joseph Boncore, of Winthrop, conceded the state has responded to some requests — which include emails to and from registry officials — but stated no further documents or electronic files had been turned over since July 29. That was one day before the panel held a seven-hour oversight hearing that included testimony from Pollack and former Registrar Erin Devaney, who resigned days after the crash amid revelations that thousands of unprocessed out-of-state notices of infractions by Massachusetts drivers had been piling up in boxes at an RMV office.

“The Committee is losing confidence in the Administration’s willingness to cooperate fully,” the letter stated.

Straus and Boncore added it would be the last time the panel “respectfully requests” the documents — strongly suggesting the committee would exercise its subpoena power if it remains unsatisfied with the response.

The lawmakers’ letter included an addendum seeking additional information, including correspondence from several non-RMV officials within the administration.

State officials deny any stonewalling.

In an emailed statement, Jacquelyn Goddard, a spokeswoman for the Massachusetts Department of Transportation, said the agency “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.”

Baker, a Republican, and Pollack have both said they were unaware of years of chronic problems with processing out-of-state notifications until after the New Hampshire crash. Baker called the failures “completely unacceptable,” and ordered an independent audit of the agency, which is ongoing.

In her testimony at the July 30 hearing, Pollack assured lawmakers the administration would continue working to provide the materials sought, noting the process involved reviewing more than 2 million emails and 6 million pages of documents.

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Volodymyr Zhukovskyy, 23, of West Springfield, Mass., the driver of a pickup truck in a fiery collision on a highway in Randolph, N.H., that killed seven motorcyclists, stands with his attorney Donald Frank during a hearing June 24 in Springfield District Court in Massachusetts. Don Treeger/The Republican via Associated Press

Zhukovskyy, 23, has pleaded not guilty to negligent homicide in connection with the New Hampshire crash, and his lawyer has denied his client was intoxicated when arrested May 11 in Connecticut.

In motions filed Wednesday, prosecutors in New Hampshire asked a judge give them until Oct. 19 to seek indictments against Zhukovskyy as well as additional time to gather evidence.

In an inspection report following the New Hampshire crash, the Federal Motor Carrier Safety Administration said toxicology findings showed Zhukovskyy tested positive for an unspecified drug that made him incapable of driving safely when his pickup truck crossed the center yellow line in Randolph, New Hampshire, and struck the bikers.

The victims were members of the Jarheads, a New England motorcycle club that includes Marines and their spouses.