The co-chairs of the state legislature’s transportation committee wrote an excoriating letter to Massachusetts Transportation Secretary Stephanie Pollack on Tuesday, saying they are "losing confidence" in the Baker administration's willingness to fully cooperate with their investigation into dangerous lapses at the Registry of Motor Vehicles.

The letter from House co-chair Rep. William Straus and co-chair Sen. Joseph Boncore says their committee has "strong reason to conclude" they received only partial responses to requests for records over the past three weeks, and demands that MassDOT hand over the records immediately.

Straus told WGBH News this is a unique situation.

“I've had the responsibility to be the transportation chair for nearly eight years — before that, environmental affairs, and before that, election laws,” Straus said, “and I've never had to write a letter like this, either as a legislator or as a legislative chair.”

The committee is investigating the RMV's failure to suspend the licenses of thousands of drivers. The investigation was prompted by a crash in New Hampshire on June 21 where a Massachusetts truck driver allegedly drove into a group of motorcyclists, killing seven people. The driver had an OUI arrest in Connecticut a month before the crash, which should have led the RMV to suspend his license.

It has since come to light that in 2016, an RMV official discovered a three-year backlog of around 10,000 printed out-of-state notices of driving violations. A more recent backlog involved dozens of storage boxes discovered in the RMV’s Quincy headquarters which, when processed, resulted in the suspension of over 1,600 drivers licenses last month.

A spokesperson for MassDOT said in a statement the department "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."

Straus said this is the last time the committee will simply request the records they seek, which include "documents on personal electronic devices and emails to/from private email accounts used by any officer and employee of the Commonwealth in the exercise of their official duties," according to the letter.

“The further option is to seek authorization from the respective chambers, House and Senate, for the committee to be able to issue subpoenas," said Straus, who added he prefers that doesn't happen.

WGBH News' Kaitlyn Locke contributed to this report.