Prosecutors in the US state of Georgia are investigating attempts by former President Donald Trump to overturn results from last November's election.
Mr Trump was recorded telling the state's top election official to "find" more than 11,000 votes, enough to give him victory there.
The official is heard replying that Georgia's results are correct.
Joe Biden's win in Georgia and other swing states secured him the presidency.
Fulton County District Attorney Fani Willis sent a letter asking state officials to preserve documents including those relating to the phone call and saying that a criminal investigation was being carried out.
"I just want to find 11,780 votes," Mr Trump told Georgia's Republican Secretary of State Brad Raffensperger in a recording of the call released by the Washington Post last month.
Donald Trump's phone call fact-checked
In the months after the election, Mr Trump made unsubstantiated allegations of widespread electoral fraud and claimed that votes were stolen from him.
The call was made on 2 January, just four days before the US Congress confirmed Mr Biden's election victory.
What does the letter say?
Ms Willis said in the letter, addressed to Georgia Governor Brian Kemp, that the investigation included:
potential violations of state law prohibiting the solicitation of election fraud
the making of false statements to state and local governmental bodies
conspiracy, racketeering, violation of oath of office
any involvement in violence or threats related to the election's administration
"This letter is notification that all records potentially related to the administration of the 2020 general election must be preserved, with particular care being given to set aside and preserve those that may be evidence of attempts to influence the actions of persons who were administering that election," she added.
"I know we all agree that our duty demands that this matter be investigated and, if necessary, prosecuted, in a manner that is free from any appearance of conflict of interest or political considerations."
Mr Raffensperger's office opened its own investigation into the phone call on Monday.
What happened during the Trump call?
In excerpts of the 2 January phone call, Mr Trump can be heard alternately cajoling and pressurising Georgia's secretary of state.
He insisted that he had won the election in Georgia and told Mr Raffensperger that there was "nothing wrong with saying you have recalculated".
Mr Raffensperger responded by saying: "The challenge you have, Mr President, is that the data you have is wrong."
Later in the call, Mr Trump said the rumour was that ballots had been shredded and voting machinery had been removed from Fulton County in the state - claims denied by Mr Raffensperger's lawyer.
The president then threatened the official with possible legal consequences.
"You know what they did and you're not reporting it. That's a criminal offence. You can't let that happen. That's a big risk to you and to Ryan, your lawyer," Mr Trump said.
He then called for the extra 11,780 votes - which would have given him a total of 2,473,634 votes in the state, one more than Mr Biden, who received 2,473,633 votes.
The president told Mr Raffensperger he should re-examine the result in the state.