The indictment of Donald Trump marks an unprecedented development in the country's history -- the first time a former president has ever faced criminal charges.

Historians say that not since Richard Nixon had there been the real prospect of a commander-in-chief being formally accused of a crime, though Nixon avoided that fate after being pardoned by successor Gerald Ford.

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-ABC News' Tal Axelrod

Republican National Committee Chairwoman Ronna McDaniel called the indictment "a blatant abuse of power from a DA focused on political vengeance."

"When our justice system is weaponized as a political tool, it endangers all of us," she tweeted.

Criminal prosecution proceedings typically start with an arrest and a court appearance, but legal experts say that on many occasions, especially in white-collar crimes, suspects aren't hit with charges or a visit from an officer until long after an official investigation is underway.

Typically, if a crime is being investigated, law enforcement agents will make an arrest, file initial charges and bring a suspect to be arraigned in court, Vincent Southerland, an assistant professor of clinical law and the director of the criminal defense and reentry clinic at NYU School of Law, told ABC News.

However, Southerland noted that prosecutors can start with the criminal indictment process in the beginning, especially if their case needs more evidence to press those charges.

Cheryl Bader, an associate clinical professor of law at Fordham University, told ABC News that such a move is common in white-collar criminal investigations that involve looking at delicate nuances in the state law and require more time.

Bader said investigations into prominent figures, such as the current investigation by the Manhattan district attorney's office into former President Donald Trump, also prompt prosecutors' offices to make their case to the grand jury in the most meticulous and thorough way possible.

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-ABC News' Ivan Pereira

The Democratic National Committee said in a statement Thursday, “No matter what happens in Trump’s upcoming legal proceedings, it’s obvious the Republican Party remains firmly in the hold of Donald Trump and MAGA Republicans."

The DNC vowed, "We will continue to hold Trump and all Republican candidates accountable for the extreme MAGA agenda that includes banning abortion, cutting Social Security and Medicare, and undermining free and fair elections.”

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.