This invitation was expressed in the Helsinki press conference (27 mins until 32 mins 30 secs). Mr Putin referenced the 1999 US / Russia agreement that allows each jurisdiction to gain access to each other’s citizens. He invited the Mueller investigation to use this agreement and his government will treat any request seriously. Mr Putin also wished to question American officials regarding Bill Browder who he claimed made $1.5 billion in Russia and paid no tax and then contributed $400 million to the Hillary Clinton campaign.
Overall I was quite heartened by the tone of the press conference with both leaders looking forward to respectfully increasing the scope of relations and peace into the future.
Full press conference :