A special grand jury is secretly hearing from witnesses in the Manhattan DA's Trump investigation. Here's how it'll decide whether to bring criminal charges.

• The Manhattan DA's office convened a "special grand jury" in its Trump investigation last month.
• Jurors will decide whether to criminally charge Trump, the Trump Organization, or its executives.
• Here's a breakdown of how a special grand jury works, as well as what evidence and witnesses will likely come before jurors as they meet in secret.

In a courtroom in downtown Manhattan, the district attorney's office is marshaling paperwork and secretly whisking witnesses in front of a "special" grand jury for what may be one of the most consequential criminal cases in the history of the United States.

The grand jury is "special" in two ways. It'll be convened for six months, rather than one month, which is standard. It also could be asked to do something no one has ever done before: indict a former president.

The Manhattan District Attorney's investigation into Donald Trump's and the Trump Organization's finances appears to have reached an advanced stage. Prosecutors are expected to announce whether they're bringing any criminal charges before the end of the year, and the special grand jury - a group of 23 ordinary citizens - will be the ones to make the final call.

Here's how the process works.

What does a special grand jury do?

The precise rules for grand juries and criminal charges vary by state. Under New York law, which governs the Manhattan District Attorney's office, a felony charge - a criminal charge that would result in a year or more in prison - may go to trial only if a grand jury decides to file an indictment.

A grand jury in New York consists of 23 people. Sixteen jurors must be present in order to make charging decisions, and 12 must vote in favor of bringing an indictment. They must meet in secret and are not permitted to discuss the case with anyone outside the jury. Grand juries can also issue subpoenas and compel witnesses to testify.

Most grand juries meet for one month and hear multiple cases. According to The Washington Post, the Manhattan DA has empaneled a "special" grand jury that will last up to six months in order to hear evidence from the Trump investigation and decide whether to bring an indictment.

Special grand juries are typically empaneled to review evidence for more sophisticated cases like Trump's, which spanned two years and involves possibly millions of pages of documents. Court documents suggest that the office of Manhattan DA Cyrus Vance Jr. has been investigating whether the Trump Organization, its executives, or Donald Trump himself broke tax laws by keeping two sets of books. One set would have shown assets of little value, in order to pay little in taxes; another would have shown significant profits in order to receive favorable loan and insurance rates. Legal experts expect Vance to make a case for tax, bank, and insurance fraud charges.

A judge may extend the special grand jury beyond the allotted six months, though Vance is widely expected to announce a charging decision before he retires at the end of December. The special grand jury, like regular grand juries, may also hear more than one case depending on how Vance's office decides to schedule it.

Grand juries are meant to function as a check on government power. Prosecutors need to convince a majority of jurors that there's probable cause to charge someone with a crime; only then can a case go to trial.

"The grand jury was designed to say, 'Hey, wait a minute, government, before you just start taking anybody you feel like to trial and convicting them and tossing them into jail or cutting their heads off, we're going to make you present evidence first and satisfy a group of people to show that we have a case,'" said Randy Zelin, an attorney at Wilk Auslander LLP and former prosecutor.

But in reality, defense lawyers like Zelin say, grand juries almost always agree with prosecutors.

In a trial, a judge oversees the process, defense lawyers can put up a defense or call their own witnesses, and prosecutors must overcome the challenge of getting a jury to reach a unanimous verdict.

But prosecutors determine which witnesses to bring and what evidence to show a grand jury - and the verdict doesn't need to be unanimous.

What evidence will jurors review?

Earlier this year, Vance's office won what's likely the biggest prize in their investigation. After two Supreme Court decisions, the district attorney's office was finally able to obtain the Trump Organization's tax documents in February.

Those documents include Trump's tax returns, which he has fought vociferously to keep from the public and which he has repeatedly lied about. Legal experts say the documents also likely include email communications between Trump Organization officials, insurance brokers, third-party accountants, and banks about what would go into those tax returns. If the communications show that Trump Organization officials sought to distort the true value of the company's properties, that could amount to tax and wire fraud, according to the legal experts.

Prosecutors have been examining that documentation for months. Now, they will present those documents as exhibits before the grand jury.

Aside from the subpoenaed tax documents, prosecutors may present jurors with financial documents provided by cooperating witnesses.

Jennifer Weisselberg, the former daughter-in-law of Trump Organization CFO Allen Weisselberg, obtained seven boxes of documents as part of an acrimonious divorce from Barry Weisselberg, Allen's son. She gave those documents to prosecutors in the fall, and in interviews with Insider said they may include evidence that Allen Weisselberg distorted employee compensation in a way that may break tax laws.

Barry Weisselberg is also a key Trump Organization employee, as the manager of the cash-only Wollman rink in Central Park. Prosecutors may also be examining documentation about the rink's operations for potential tax fraud, according to people familiar with the investigation.

The financial documents investigators gathered may be complex and difficult for a grand jury to comprehend. That poses a challenge for Vance's office. Under New York state law, prosecutors can't present "hearsay evidence," or prosecutors' personal summaries of the evidence, to grand juries. Instead, prosecutors must provide the underlying documentation directly to the jurors.

Prosecutors can also get third parties to present evidence, including charts and summaries, for them. Vance's office is working with FTI Consulting, a forensic accounting firm, to analyze Trump's documents, and those accountants may present their analysis to a grand jury.

"There could be summary charts or analyses that are put together by the person who has done that analysis to help the grand jury understand the documents," said Rebecca Ricigliano, a former first assistant attorney general for the state of New Jersey and longtime federal prosecutor in Manhattan. "You see that all the time in regular trials where there's complicated financials or complicated issues concerning voluminous documents."

What witnesses will jurors hear from?

The person or entities under investigation don't have any say in who's called as a witness before a grand jury. It's up to prosecutors to decide who testifies.

By default, anyone who gets called as a witness in front of a grand jury in New York gets "transactional immunity," meaning they get total immunity from prosecution for any possible crimes related to their testimony. Transactional immunity protects people from being subpoenaed and forced to incriminate themselves.

It also means prosecutors need to secure cooperating witnesses before going to a grand jury. The names of some of those witnesses in the DA's investigation are already public.

Michael Cohen, a former Trump Organization executive and personal lawyer for the ex-president, told Insider he's spoken to prosecutors more than a dozen times. Jennifer Weisselberg has helped prosecutors understand the company's inner workings, which she's described as corrupt. Both will likely serve as grand jury witnesses, and both declined Insider's request to discuss their possible role, saying they don't want to jeopardize the legal process.

On Friday, ABC News reported that Vance's office brought Jeff McConney, a senior vice president and controller for the Trump Organization, to testify in front of the special grand jury.

Prosecutors also may have secured less boldfaced names, like tax preparers at Mazers, the firm used by the Trump Organization, or officials at the insurance firm Aon.

It's unclear whether prosecutors have secured cooperation from Allen Weisselberg, who is widely viewed as a potential key witness for the case. He's worked for the Trumps for more than four decades, and knows the company's as well as the family's finances inside and out. Prosecutors reportedly want him to testify about those finances.

But prosecutors wouldn't want to just call Weisselberg as a grand jury witness, which would give him immunity from prosecution for any financial crimes he may have committed in his role as CFO.

Ricigliano noted that people shouldn't assume Weisselberg will make or break the DA's case.

"It's hard to judge whether he's really a make-or-break figure, or if they have been able to compile the evidence they need from multiple other sources," Ricigliano, now an attorney at Crowell & Moring, told Insider. "You can build a case like building a house. You can either get a crane to drop down a prefab house, or you can build it brick by brick."

Does Trump himself play any role?

If Trump himself is a subject of the DA's probe, he wouldn't be called as a grand jury witness. If prosecutors want to charge him, giving him transactional immunity would defeat the purpose of their investigation.

Still, if a lawyer knows their client is a subject of a probe, they can serve a "grand jury notice" to the district attorney's office, Zelin told Insider.

If that notice is served, prosecutors may be forced to inform the special grand jury that a particular witness named by the defense attorney is willing to testify. The jurors may then decide to subpoena the witness, giving that person transactional immunity against the prosecutors' wishes.

"It's sometimes a sophisticated and savvy move on a defense attorney's part," Zelin said.

To head off that scenario, prosecutors will try to make a deal with potential witnesses - called a cooperation or plea agreement - where they'd testify in front of a grand jury and agree to waive transactional immunity.

"Prosecutors are very strategic in thinking through their presentation of evidence to ensure that they are not immunizing a witness who they shouldn't be immunizing," Ricigliano said.

What charging decisions can the special grand jury make?

It's not clear whether Vance will seek to charge Trump, Trump Organization executives, the company - or not bring charges at all.

Former New York Supreme Court chief judge Sol Wachtler said a grand jury would "indict a ham sandwich" if a prosecutor told them to because of the amount of control they have over the process. Zelin said because of that dynamic, grand juries can offer prosecutors political cover.

If prosecutors don't want to pursue a politically inconvenient case, they can blame the grand jury for not bringing an indictment.

But grand juries can also offer protection: Trump has already attempted to tarnish Vance's investigation by describing it as politically motivated. If a grand jury brings an indictment against Trump, Vance could persuasively say that fellow citizens, not just his office, believe the former president may have committed a crime.

Any charging decisions in this case will be controversial. Democrats sore over Trump's apparent ability to wriggle out of every scandal will be further rattled if he isn't personally charged. Trump supporters may be scandalized if he's held responsible for financial wrongdoing that may have been engineered by other executives like Weisselberg.

Daniel R. Alonso, a former top Vance deputy, previously told Insider there's a good chance the DA may ask the grand jury to bring charges against only the Trump Organization.

Charging the Trump Organization alone would be a smart strategic move that would further puncture Trump's image as a successful businessman, possibly cut into his family's reputation, and provoke less backlash from the 74 million people who voted for him, Zelin said.

"An argument could be made that it's more devastating," Zelin said. "If you kill the Trump Organization, basically you kill the Trump family's means and a rather extravagant high-profile and lavish lifestyle."

Ricigliano told Insider that prosecutors don't typically think about political considerations when they bring cases in front of a grand jury.

She emphasized that prosecutors in Vance's office - as well as prosecutors with New York Attorney General Letitia James' parallel investigation - gathered massive amounts of information over the course of their two-year probe, and the public only is aware of a small fraction of it.

Those prosecutors will ultimately bring the best case they have based on the evidence, she said.

"I don't think that prosecutors view the grand juries as potential scapegoats or ways to alleviate pressure," Ricigliano said. "It's part of the system that's been part of the country since the dawn of the revolution."