WASHINGTON — Special counsel Robert Mueller’s investigation into Russian contacts with President Trump’s campaign has gained access to thousands of emails sent and received by Trump officials before the start of his administration, the general counsel for Trump’s transition organization says.

Mueller’s investigators obtained the emails from the General Services Administration, a federal agency that stored the material, rather than request them from Trump’s still-existing transition group, Trump for America, Kory Langhofer said in a letter sent to two congressional committees.

Langhofer said the GSA had improperly provided the transition records to Mueller’s team, which he said has been “actively using” the emails. Langhofer told Republican chairmen of the House Oversight and the Senate Homeland Security committees that the disclosure by GSA was “unauthorized,” and it considers the documents private and privileged and not government property.

Trump on Sunday criticized the fact that Mueller had gained access to thousands of emails sent and received by Trump officials before the start of his administration. He said it was “not looking good” and again stressed that there was “no collusion” with Russia – an important question the probe is examining.

The documents were provided to Mueller’s team by the GSA in September in response to requests from the FBI, but the transition team didn’t learn about it until last week, Langhofer said.

The tens of thousands of emails pertain to 13 senior Trump transition officials. Many of the emails that Mueller’s investigators have now include national security discussions about possible Trump international aims as well as candid assessments of candidates for top government posts, said those familiar with the transition.

Langhofer also said that a GSA official appointed by Trump in May had assured the transition in June that any request for records from Mueller’s office would be referred to the transition’s attorneys. According to Langhofer, the assurance was made by then-GSA General Counsel Richard Beckler, who was hospitalized in August and has since died.

But late Saturday, another GSA official present for the conversation told Buzzfeed News that there was nothing improper about the disclosure of the emails to Mueller’s team. The GSA has provided office space and other aid to presidential transitions in recent years and typically houses electronic transition records in its computer system.

GSA Deputy Counsel Lenny Loewentritt, who Langhofer blames along with other GSA career staff for providing the transition documents to the FBI, told Buzzfeed that Beckler didn’t make a commitment to the transition team that requests from law enforcement for materials would be routed through transition lawyers.

Transition officials signed agreements that warn them that materials kept on government servers are subject to monitoring and auditing, Loewentritt told Buzzfeed, and there’s no expectation of privacy.

Late Saturday, Mueller’s spokesman, Peter Carr, said the special counsel’s office has followed the law when it has obtained documents during its investigation.

“When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner’s consent or appropriate criminal process,” Carr said.

In a statement, Rep. Elijah Cummings of Maryland, the ranking Democrat on the House Oversight and Government Reform committee, dismissed the transition’s arguments that GSA shouldn’t have turned over the records to Mueller.

Among the officials who used transition email accounts was former National Security Adviser Michael Flynn, who pleaded guilty to a count of making false statements to FBI agents in January and is cooperating with Mueller’s investigation. Trump fired Flynn in February for misleading senior administration officials about his contacts with Russia’s ambassador to the U.S.

Flynn attorney Robert Kelner declined to comment. Jay Sekulow, an attorney on Trump’s personal legal team, referred questions to the transition group. Spokespeople for GSA didn’t respond to AP’s emailed requests for comment.

It’s unclear how revelatory the email accounts maintained by the GSA will be for Mueller. Several high-level Trump advisers sometimes used other email accounts, including their campaign accounts, to communicate about transition issues between Election Day and the inauguration.

The special counsel’s office also obtained at least one iPad as well as laptops and cellphones that were used by the transition, but prosecutors have assured the transition that investigators have not pulled emails and other data from those devices, Langhofer said. He did not name the transition officials who used the devices.

The media site Axios first reported on the transfer of the emails to Mueller’s team.