Several former intelligence and FBI officials cooperating in a United States Department of Justice investigation into former John Brennan have been subpoenaed to testify before a grand jury in Washington, D.C..

The case examines allegations that Brennan lied to Congress about the Central Intelligence Agency’s role in the 2016 Russia election interference assessment.

The subpoenas came after a change in leadership, with a Trump ally, Joe DiGenova, taking over the investigation following the removal of a career prosecutor who reportedly raised concerns about the evidence.

Legal experts say the shift to grand jury testimony is unusual and may reflect efforts to strengthen a politically sensitive case, CBS News has reported.

Brennan’s attorney denies any wrongdoing, and some prior witness testimony has not supported the allegations.