WASHINGTON (AP) — Special counsel Robert Mueller’s investigation into Russian contacts with President Donald Trump’s campaign has gained access to thousands of emails sent and received by Trump officials before the start of his administration, according to several people familiar with Trump’s transition organization.

But the investigators did not directly request the records from Trump’s still-existing transition group, Trump for America, and instead obtained them from the General Services Administration, a separate federal agency that stored the material, according to those familiar with the Trump transition organization.

The tens of thousands of emails in question 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. They spoke to The Associated Press on condition of anonymity because of the records’ sensitivity.

On Saturday, Kory Langhofer, general counsel for the transition group, sent a letter to two congressional committees arguing that the GSA had improperly provided the transition records to Mueller’s investigators. In the letter to the Republican chairmen of the House Oversight and the Senate Homeland Security committees, Langhofer contends that the disclosure by GSA was “unauthorized,” and it considers the documents private and privileged and not government property.

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. A copy of the letter was viewed by the AP.

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 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.

Loewentritt said the transition was informed that by using government devices, the agency wouldn’t hold back records from law enforcement. Transition officials signed agreements that warn them that materials kept on the government servers are subject to monitoring and auditing, he told Buzzfeed, and there’s no expectation of privacy.

The documents were provided to Mueller’s team by the GSA in September in response to requests from the FBI, but the transition wasn’t informed at the time, according to people familiar with the transition organization. Officials with Trump for America learned last Wednesday that GSA officials had turned over the cache of emails to Mueller’s team.

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.

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 now cooperating with Mueller’s investigation. Flynn was fired by Trump in February for misleading senior administration officials about his contacts with Russia’s ambassador to the U.S.

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 to communicate about transition issues between Election Day and the inauguration.

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 immediately respond to AP’s emailed requests for comment.

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

By STEPHEN BRAUN and CHAD DAY