Former President Donald Trump is expected to travel to New York on Monday, sources familiar with the matter told ABC News.

He is expected to appear in court on Tuesday at the earliest, the sources said, on what is expected to be around two dozen counts - including felonies.

The exact charges are unknown since the indictment will not be unsealed until Trump appears in court.

-ABC News’ Aaron Katersky, Katherine Faulders, John Santucci

As legal experts speculate on what charges lay inside the sealed indictment ahead of former President Donald Trump's expected surrender on Tuesday afternoon, many predict that prosecutors could try out a new legal theory to justify bringing the charges.

"This could be a novel legal theory," said Kate Shaw, a law professor at Cardozo and ABC News contributor, speculating on what charges the public could see against Trump while stressing it's unknown until the indictment is unsealed.

Read more here.

-ABC News' Libby Cathey

Former President Donald Trump has been charged with around two dozen counts, including felonies, sources familiar with the sealed indictment told ABC News.

The exact charges remain unknown since the indictment will not be unsealed until Trump appears in court on Tuesday.

The Manhattan district attorney’s office declined to comment.

The Senate sergeant at arms is warning of potential demonstration activity related to the indictment of former President Donald Trump.

"While law enforcement is not tracking any specific, credible threats against the Capitol or state offices, there is potential for demonstration activity," an email obtained by ABC News said.

Capitol Police "is working with law enforcement partners, so you may observe a greater law enforcement presence on Capitol Hill," the email said, adding that there could be "nationwide impacts to Senate state offices."

The Capitol Police declined to comment and the sergeant at arms didn't immediately respond to ABC News' request for comment.

-ABC News' Luke Barr

A white powder was discovered in the mailroom at 80 Centre Street, where the Manhattan District Attorney has offices and where a grand jury has been meeting to hear evidence in former President Donald Trump's case, according to a court official. The powder was determined to be non-hazardous, officials said.

The powder came in an envelope addressed to "Alvin," an apparent reference to Manhattan District Attorney Alvin Bragg, according police sources.

Inside the envelope was a letter containing the typewritten message, “Alvin: I am going to kill you," with 13 exclamation points, according to sources.

This envelope followed a series of unfounded threats that targeted municipal offices in New York this week.

"For three days we got four emails," Susan Stetzer, district manager at Manhattan Community Board 3, told ABC News on Friday.

At least one of the messages prompted the court to pause a hearing in the New York Attorney General’s civil lawsuit against Trump.

None of the email messages mentioned Trump by name. One included what Stetzer described as a "horrible homophobic rant."

According to Stetzer, the messages came from @mail.ru domains and some contained Cyrillic characters. The FBI is aware but does not immediately assess that the emails came from Russia, according to a law enforcement official.

"We did not get one today so I’m hoping it stops," Stetzer said.

New York City courthouses will see increased security, the Office of Court Administration said Friday.