A Georgia prosecutor investigating possible criminal behavior by former President Donald Trump in the 2020 presidential election has officially requested to seat a special grand jury, according to a letter obtained by ABC News.

       The development is a major step forward in the only publicly known criminal investigation into Trump's efforts to overturn the results of the 2020 election.

       In a letter Thursday to Fulton County Chief Judge Christopher Brasher, Fulton County District Attorney Fani Willis wrote that the move is needed because "a significant number of witnesses and prospective witnesses have refused to cooperate with the investigation absent a subpoena requiring their testimony."

       Willis officially launched the probe last February, after Trump was heard in a recorded phone call pushing Georgia Secretary of State Brad Raffensperger to help him "find 11,780 votes," the exact number Trump needed to win Georgia in the 2020 presidential election.

       MORE: Fulton County DA opens criminal probe into Trump's efforts to overturn Georgia election results

       Willis says that Raffensperger is one of those who will not comply with the investigation without a subpoena, based on comments he made in an interview with NBC.

       In response to Willis' request, Trump, in a statement, said, "My phone call to the Secretary of State of Georgia was perfect, perhaps even more so than my call with the Ukrainian President, if that’s possible." The reference was to the phone call Trump made to Ukrainian President Volodymyr Zelensky ahead of the 2020 election asking him to dig up dirt on his political rival Joe Biden; Trump was ultimately impeached for that call, but the Senate did not convict him.

       Mandel Ngan/AFP via Getty Images, FILE

       In this Jan. 4, 2021, file photo, President Donald Trump gives two thumbs up during a rally in support of Republican incumbent senators Kelly Loeffler and David Perdue ahead of Senate runoff in Dalton, Ga.

       "I didn’t say anything wrong in the call," Trump said of his call to Raffensperger. "No more political witch hunts!"

       If empaneled, the special grand jury will not have the authority to return an indictment, according to the Willis' letter. Instead it may "make recommendations concerning criminal prosecution as it shall see fit," the letter said.

       MORE: Trump demands Georgia secretary of state 'find' enough votes to hand him win

       A majority of the judges on the Fulton County Superior Court will have to vote to approve the request in order for the special grand jury to be seated, according to Georgia state law.

       Describing his Jan. 2 call with Trump in an exclusive interview with ABC News' George Stephanopoulos last year, Raffensperger said that Trump "did most of the talking."

       "We did most of the listening," Raffensperger said. "But I did want to make my points that the data that he has is just plain wrong."

       ABC News' Steve Osunsami and Brandon Baur contributed to this report.