ATLANTA • The prosecutor for Georgia’s biggest county on Thursday requested a special grand jury with subpoena power to aid her investigation into then-President Donald Trump’s efforts to influence the U.S. state’s 2020 election results.

In a letter to Fulton County’s chief judge, first reported by the Atlanta Journal-Constitution, District Attorney Fani Willis wrote that multiple witnesses being probed have refused to cooperate absent a subpoena requiring their testimony.

“Therefore, I am hereby requesting … that a special purpose grand jury be impaneled for the purpose of investigating the facts and circumstances relating directly or indirectly to possible attempts to disrupt the lawful administration of the 2020 elections in the State of Georgia,” Willis wrote.

The investigation by Willis, a Democrat, is the most serious probe facing Trump in Georgia after he was recorded in a phone call pressuring Georgia Secretary of State Brad Raffensperger to overturn the state’s election results based on unfounded claims of voter fraud.

Willis specifically mentioned that Raffensperger, whom she described as an “essential witness,” had indicated he would only take part in an interview once presented with a subpoena.