Comey was approved by the Justice Dept to testify before the House committee.
But it wasn't Jeff Sessions, the Attorney General. It was the Deputy Attorney General Dana J. Boente, who was appointed by Barack Obama.
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https://www.justice.gov/dag/staff-profile/meet-acting-deputy-attorney-general
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Dana J. Boente was named Acting Deputy Attorney General of the United States on February 9, 2017. Prior to his current appointment, Boente served as Acting Attorney General of the United States from January 30 to February 9, 2017.
Boente was nominated by President Barack Obama on October 8, 2015, and confirmed by the U.S. Senate on December 15, 2015, as the 60th U.S. Attorney for the Eastern District of Virginia. As the chief federal law enforcement for the District, which includes offices in Alexandria, Richmond, Norfolk and Newport News, Boente supervises the prosecution of all federal crimes and the litigation of all civil matters in which the federal government has an interest.