Christopher Steele, the former British intelligence officer and author of the original Trump dossier, has reached an agreement with the U.S. Justice Department to testify in support of the department’s investigation into FBI counterintelligence activities targeting President Trump and his campaign staff.

Steele will be interviewed by John Durham, U.S. attorney in Connecticut, whom Attorney General William Barr has appointed to lead the investigation and to serve as federal prosecutor. Attorney General Barr seeks to determine whether FBI activities to investigate the Trump campaign were “lawful and appropriate.”

The FBI made use of Steele’s dossier during the early stages of its investigation into the Trump campaign. This occurred at the same time that Steele was being paid to produce the dossier by the Democratic National Committee and Hillary Clinton’s presidential campaign. According to a State Department memo, written by Deputy Assistant Secretary Kathleen Kavalec, Steele also held a meeting with a high-level official at the State Department in October 2016, during which he admitted he was working on behalf of a political client who was “keen to see this information come to light prior to November 8,” the date of the 2016 election.

Steele subsequently served as a confidential informant for the FBI when it obtained the first of several Foreign Intelligence Surveillance Act (FISA) warrants used to place a Trump campaign aide under surveillance.

To date, it remains unclear how credible Steele’s dossier is, or whether the information it contains—including salacious details it alleges about the president’s personal life—was independently verified.

Questions also linger as to why the special counsel’s investigation into the 2016 election continued on for nearly two years after it failed to discover evidence of a conspiracy between Trump’s campaign and Russia. Steele’s testimony is expected to help reveal whether there existed a faction within the FBI that acted in support of its own agenda to try to incriminate the president.