Prosecutors have been using previously empaneled grand juries to issue subpoenas and gather evidence in an investigation that has spanned the better part of two years.
Grand juries only sit for a few months at a time. A special grand jury sits for a longer period of time. This one is expected to sit for six months, and three days at a time.
Empaneling a special grand jury suggests the case has reached an advanced stage, but as yet, there have been no charges filed.
Potential witnesses have been contacted in recent weeks about appearing before the special grand jury, the sources told ABC News.
Add that to the news that New York Attorney General Letitia James now has her own criminal investigation with investigators working together with Vance's office, and experts say all this is significant.
"These investigations take a lot of time and it is going to take a while for the grand jury to digest all the material," said Mike Sisak, Associated Press. "And in this investigation some of that material includes Donald Trump's tax returns and other records related to his finances."
In a statement late Tuesday, the former president said in part:
"This is a continuation of the greatest Witch Hunt in American history. It began the day I came down the escalator in Trump Tower, and it's never stopped...No other President in history has had to put up with what I have had to...Our Country is broken, our elections are rigged, corrupt, and stolen, our prosecutors are politicized."
Some think Trump will consider this a win.
"He will use this to his advantage, Trump will. Why? He will say he's a victim of prosecutors who are overzealous and who are trying to destroy the people that he represents," political consultant Hank Sheinkopf said.
Word of the special grand jury's existence was first reported by the Washington Post.
Manhattan DA Cy Vance began investigating Trump's business practices based on the Congressional testimony of Michael Cohen, who served as the former president's personal attorney and fixer.
One focus of the investigation includes whether Trump inflated the value of certain properties to obtain bank loans and deflated the value of those same properties to pay lower taxes.
"All he needs to say now is by the way they're trying to stop our revolution," Sheinkopf said. "They stole the election from us and now they're trying to put us in jail. Don't let them get away with it."
Vance fought all the way to the U.S. Supreme Court twice to gain access to eight years' worth of Trump's personal and business tax returns.
Vance is not running for re-election, so this will be "his" final word.
"Whether he gets a conviction or not, Cy Vance has his legacy in tact," Sheinkopf said. "Why? He tried to put away the guy that New Yorkers hate worse than anyone else."
RELATED | Investigation of Trump's business looms over Manhattan DA race
* Get Eyewitness News Delivered
* More Manhattan news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube
Submit a News Tip