Senior American officials and military officers are being quizzed in secret by the official inquiry into Britain's role in the Iraq war.
Sir John Chilcot's committee is spending five days in Washington and Boston interviewing members of the administrations of President Bush and President Obama.
Suggestions that Tony Blair secretly agreed with President Bush to join the war more than a year before the invasion are likely to be one of the key areas of interest to the inquiry.
Sir Christopher Meyer, the former British Ambassador to Washington, has told the inquiry that the Prime Minister "signed in blood" a deal to overthrow Saddam Hussein during a private meeting at the President's ranch in Texas.
Some senior military officers have told the inquiry that their American counterparts were convinced British forces would join the war while politicians in London were still publicly insisting that no decision had been reached.
An inquiry spokesman said: "As the talks are being held on a private basis, the identities of the people the Inquiry committee are seeing and the location of meetings will not be revealed in advance.
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"Subject to the agreement of participants, the Inquiry may provide more details about the trip after it has been completed. If the committee wishes to use any of the information it receives from individuals in America in its report, it will seek their permission first."
The inquiry team has said that no transcripts of the evidence will be made public.
The Chilcot committee travelled to Paris earlier this month, where it is believed to have attempted to clarify if France would have supported United Nations involvement in toppling Saddam.
Mr Blair told the inquiry that Britain had to go to war without a second United Nations resolution authorising the action because France had made it "vehemently" clear that it would never support such a move.
Witnesses questioned by the inquiry included Dominique de Villepin, the former French Minister of Foreign Affairs; Jean-David Levitte, the French Permanent Representative to the UN from 2000 to 2002 and then Ambassador to the US from 2002 to 2007; and Gérard Errera, the French Ambassador to London from 2002 to 2007.
The committee has also interviewed Maurice Gourdault-Montagne, the senior diplomatic counsellor to the President between 2002 and 2007, and currently the French Ambassador to the UK.