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.

Former President Donald Trump has escalated his attacks on Manhattan District Attorney Alvin Bragg and his staff. Overnight, Trump posted on social media that if he were to be indicted it could result in "potential death and destruction."

Senior administration officials at the Department of Homeland Security are continuing to "watch closely, particularly in the online environment" surrounding a potential indictment against former President Donald Trump, a senior administration official said.

There is nothing "that rises to the level of being credible and specific" or "actionable," the administration official said. However, the official said that online "there are always things that emerge that will cause people to take note and possibly raise concern."

As the grand jury continues, the lines of communication with local authorities like the NYPD and Capitol Police have been "wide open."

"It's been a several day period of, I'd say, very open and continued information exchange between and among federal and state partners, focused on this issue," a senior administration official said.

-ABC News' Luke Barr

Manhattan District Attorney Alvin Bragg’s general counsel responded to House Republicans Thursday, telling them compliance with their requests for information would interfere with a legitimate law enforcement investigation.

General counsel Leslie Dubeck noted the House inquiry only resulted from former President Donald Trump’s social media post.

“Your letter dated March 20, 2023 (the "Letter"), in contrast, is an unprecedented inquiry into a pending local prosecution," Dubeck wrote. "The Letter only came after Donald Trump created a false expectation that he would be arrested the next day and his lawyers reportedly urged you to intervene. Neither fact is a legitimate basis for congressional inquiry."

Manhattan District Attorney Alvin Bragg on Friday again resisted the House GOP investigation of his office’s prosecution of Trump.

Bragg’s legal counsel called the inquiry “illegitimate incursion” into a legitimate investigation.

The House Republicans -- Reps. Jim Jordan, James Comer and Bryan Steil -- have said they want to find evidence of federal funds used to investigate Trump. But Bragg’s office said they were merely doing Trump’s bidding.

“Finally, as you are no doubt aware, former President Trump has directed harsh invective against District Attorney Bragg and threatened on social media that his arrest or indictment in New York may unleash 'death & destruction.' As Committee Chairmen, you could use the stature of your office to denounce these attacks and urge respect for the fairness of our justice system and for the work of the impartial grand jury," counsel Leslie Dubeck wrote in a letter to the lawmakers.

"Instead, you and many of your colleagues have chosen to collaborate with Mr. Trump’s efforts to vilify and denigrate the integrity of elected state prosecutors and trial judges and made unfounded allegations that the Office’s investigation, conducted via an independent grand jury of average citizens serving New York State, is politically motivated.”