Special counsel Robert Mueller’s probe into Russian contacts with US 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 people familiar with Mr Trump’s transition organisation.

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

The tens of thousands of emails in question pertain to 13 senior Trump transition officials. Many of the emails Mr 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.

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 Mr Mueller’s investigators. In the letter to the Republican chairmen of the House Oversight and the Senate Homeland Security committees, Mr Langhofer contends that the disclosure by GSA was “unauthorised” and it considers the documents private and privileged and not government property.

But later, another GSA official told Buzzfeed News there was nothing improper about the disclosure of the emails.

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.