Mueller sent a notice, called a document preservation request, asking White House staff to save "any subjects discussed in the course of the June 2016 meeting" and also "any decisions made regarding the recent disclosures about the June 2016 meeting," according to the source, who read portions of the letter to CNN.
The letter from Mueller began: "As you are aware the Special Counsel's office is investigating the Russian government's efforts to interfere in the 2016 presidential election, including any links or coordination between the Russian government and individuals associated with the campaign of Donald Trump. Information concerning the June 2016 meeting between Donald J Trump Jr and Natalia Veselnitskaya is relevant to the investigation."
The preservation request is broad and includes text messages, emails, notes, voicemails and other communications and documentation regarding the June 2016 meeting and any related communication since then.
On Wednesday, the White House counsel's office sent a notice to White House staff informing them of the document preservation request. A second source confirmed to CNN the letter was sent to White House personnel.
Such notices are frequently sent in the early stages of investigation and puts those who receive it on notice of the need to hold on to the material and specifically not to destroy documents that could be relevant to the investigation and requested in the future.
A White House spokeswoman told CNN they don't comment on internal communications and the special counsel's office declined to comment.
CNN reported last week
that the White House scramble to respond to revelations about Donald Trump Jr.'s emails may have exposed the aides involved to special counsel scrutiny about what they learned about that meeting.
White House aides and Kushner's legal team began strategizing late last month over how to manage the disclosure of newly discovered emails setting up the June 2016 meeting, according to sources close to Kushner's legal team.
Their public relations efforts culminated in a series of stumbles in response to inquiries from media outlets about the June 2016 meeting.
Some of the President's closest aides, who were traveling with him back from Europe helped strategize about a response for Trump Jr., according to people briefed on the matter. The New York Times first reported on the crafting of the statement.
A sensitive legal matter such as this would normally have been handled by the attorneys, given that it was about the Russia investigation.
But the President's lawyer, Marc Kasowitz, who is designated to handle personal legal issues, was not traveling with the President and was largely uninvolved, according to the people familiar with the matter. The Times reported last week that the President himself approved the statement, raising the possibility the President may have opened himself up to new legal issues not covered by attorney-client privilege. Jay Sekulow, the President's attorney, denied that Trump was involved.
"I wasn't involved in the statement drafting at all, nor was the President. I'm assuming that was between Mr. Donald Trump Jr., between Don Jr. and his lawyer. I'm sure his lawyer was involved, that's how you do it," said Sekulow in an interview on CNN's New Day