The prosecutor for Georgia’s biggest county on Thursday requested a special grand jury with subpoena power to aid her investigation into former US president Donald Trump’s efforts to influence the state’s 2020 election results.

In a letter to Fulton county’s chief judge, first reported by the Atlanta Journal-Constitution newspaper, district attorney Fani Willis wrote that multiple witnesses have refused to cooperate without 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 inquiry facing Trump in Georgia after he was recorded in a phone call pressuring Georgia’s secretary of state, Brad Raffensperger, to overturn the states election results based on unfounded claims of voter fraud.

The prosecutor 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.

A spokesperson for Trump did not immediately respond to a request for comment. A Trump adviser previously criticized the investigation as a “witch-hunt” designed to “score political points”.

A spokesperson for the superior courts in Fulton county, which encompasses most of the state capital Atlanta, said there was no immediate timeline for a response to Willis’s request.

In the letter, Willis said a special grand jury, which can subpoena witnesses, was needed because jurors can be impaneled for longer periods and focus exclusively on a single investigation.

During the 2 January 2021 call, the former Republican president urged Raffensperger to “find” enough votes to overturn his Georgia loss to Democrat Joe Biden. The transcript quotes Trump telling Raffensperger: “I just want to find 11,780 votes,” which is the number Trump would have needed to win Georgia.

Legal experts have said Trump’s phone calls may have violated at least three state election laws: conspiracy to commit election fraud, criminal solicitation to commit election fraud, and intentional interference with performance of election duties.

The possible felony and misdemeanor violations are punishable by fines or imprisonment.