Not so fast. A New York grand jury will hear a final surprise witness on Monday and won't vote on Trump indictment until it's over.

• A Manhattan grand jury will continue hearing evidence Monday in the Trump hush-money investigation.
• A final witness must testify before the panel can deliberate and then vote on a possible indictment.

A possible Donald Trump "hush-money" indictment is on hold until a final witness testifies before a Manhattan grand jury on Monday afternoon.

"There is one more witness," a source with knowledge of the investigation told Insider on Saturday night.

The source spoke on condition of anonymity because they were not authorized to divulge details of the grand jury proceedings.

The source declined to identify the witness, whose testimony will cap a two-month grand jury presentation by prosecutors under District Attorney Alvin Bragg.

A separate source, who also spoke on condition of anonymity, told Insider that the witness is not Allen Weisselberg, Trump's former CFO, who is serving a five-month sentence for admittedly masterminding a payroll tax-dodge scheme at the Trump Organization.

News of a Monday grand jury witness was first reported by CNN.

Former Trump attorney and "fixer" Michael Cohen — the prosecution's star witness for his admitted role in wiring the illegal $130,000 payment to adult film actress Stormy Daniels, days before the 2016 election — has told reporters he expected to be the grand jury's final witness when he testified last Monday and Wednesday.

The surprise final witness provides an updated clue to the timing of a possible indictment of Trump and of any co-defendants.

The grand jury, which meets in secret in a lower Manhattan office building, only gathers to hear testimony during three-hour afternoon sessions on Mondays, Wednesdays, and Fridays.

Its members — anywhere from 16 to 23 in number — could conceivably reach a vote by the end of Monday's three-hour session.

But that would be unlikely. Experts who have described the Manhattan grand jury process to Insider say there are several steps between the final testimony and the vote.

Once testimony concludes, prosecutors will "charge" the jury, meaning go through the potential counts on a charge sheet one by one, explaining each count in the potential indictment.

Sources have told Insider they expect the top count to be falsifying business records in the first degree, a low-level felony that would allege that Trump and any other co-defendant falsified documents in order to conceal another crime, such as omitting the $130,000 from campaign financial statements.

Trump has vehemently denied any wrongdoing or having an affair with Daniels, and has called the prosecution a "Scam, Injustice, Mockery, and a Complete and Total Weaponization of Law Enforcement in order to affect a Presidential Election!"

Once the charge has ended, the prosecutor, court officer, and stenographer leave the room, and the grand jurors begin deliberating.

If 12 or more reach a vote to indict, the foreperson would be given the hard-copy indictment to sign, at which point the former president would officially, though secretly, be under indictment.

That hard-copy indictment would then be walked over to a nearby clerk's office, where it is filed under seal. It would be unsealed at Trump's arraignment, though Bragg could ask a judge to unseal it early, given the great public interest.

It's a lot of activity to fit into three hours; if the process is incomplete, the grand jurors could return on Wednesday afternoon to keep working.

Trump had "Truthed" earlier Saturday that he "will be arrested on Tuesday of next week," telling supporters "Protest, take our nation back!" But that timing, already disputed by his attorney, could never have worked given the new final witness.

The panel has so far heard from a steady stream of witnesses, including Cohen. Former Trump advisors Hope Hicks and Kellyanne Conway have also appeared.