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NEW YORK (WPIX) – Top brass at the NYPD issued a memo on Thursday instructing all officers to wear their uniforms and prepare for mobilization following the Manhattan grand jury indictment of former President Donald Trump.

The internal NYPD memo, obtained by Nexstar’s WPIX, stated that officers of all ranks should perform their duties in uniform and be prepared for mobilization beginning Friday morning.

The NYPD and other law enforcement agencies have been on heightened alert since last week in anticipation of the looming grand jury decision in the Manhattan district attorney’s case.

Last Friday, a powdery substance and a threatening letter were found inside a mailroom at DA Alvin Bragg’s offices. The NYPD and environmental protection officials isolated and removed the suspicious letter, and testing “determined there was no dangerous substance,” Bragg spokesperson Danielle Filson said.

Bragg also acknowledged in a memo to staff that the office has been receiving offensive and threatening phone calls and emails. He thanked his staff of nearly 1,600 people for persevering in the face of “additional press attention and security around our office“ and said their safety remains the top priority.

The grand jury indictment stems from District Attorney Bragg’s investigation into Trump’s alleged role in organizing hush money payments to adult film star Stormy Daniels made during the 2016 presidential campaign to silence claims of an extramarital sexual encounter.

The grand jury voted on Thursday to indict Trump, however, the charges laid out in the indictment remain unclear. It’s the first-ever criminal case against a former U.S. president and a jolt to Trump’s bid to retake the White House in 2024.

Trump, who has denied any wrongdoing and has repeatedly attacked the investigation, called the indictment “political persecution” and predicted it would damage Democrats in 2024. He is expected to surrender to authorities next week, though the details are still being worked out.

A spokesperson for Bragg said in a statement that his office has reached out to Trump’s attorney to coordinate the former president’s surrender.

“This evening we contacted Mr. Trump’s attorney to coordinate his surrender to the Manhattan D.A.’s Office for arraignment on a Supreme Court indictment, which remains under seal. Guidance will be provided when the arraignment date is selected,” the spokesperson said.

The sidewalks outside Bragg’s office were lined with police barricades on Thursday night. As security guards escorted the district attorney from the building to a waiting vehicle, someone on the street shouted “lock him up!”

The Associated Press contribued to this report.