The Daily Caller's Chuck Ross reports that Simpson and Fritsch make the admission in their upcoming book, "Crime In Progress: Inside the Steele Dossier and the Fusion GPS Investigation of Donald Trump."

The two are largely defensive of the dossier, according to the Atlantic which received an advanced copy of the book.

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"After three years of investigations, a fair assessment of the memos would conclude that many of the allegations in the dossier have been borne out," the two write in the book,  claiming that some of Christopher Steele's findings "proved remarkably prescient."

However, they write that other allegations in the dossier "remain stubbornly unconfirmed, while a handful now appear to be doubtful, though not yet disproven."

Ross writes that Simpson and Fritsch's admission is significant because they have not acknowledged any flaws in the dossier before.

The two also write about their relationships with journalists in the lead up to the 2016 presidential election and describe their Washington, D.C. offices as a "public reading room" for reporters who wished to dig up dirt on the Trump campaign.