Five envelopes containing a suspicious white powder were discovered at New York’s Immigration and Customs Enforcement field office in Manhattan Thursday afternoon – prompting first responders to evacuate the federal building, according to authorities.

The substance was later found to be boric acid, law-enforcement sources said.

Fire officials responded to 26 Federal Plaza on Foley Square just before 4 p.m. when personnel found letters containing the mysterious substance in the ninth-floor mailroom of the Jacob K. Javits Federal Office Building, officials said. 

The historic address, also home to the US Department of Homeland Security, immigration court and other federal agencies, was swiftly evacuated as the Fire Department’s hazmat teams investigated. 

The envelopes were sent to the FBI’s Quantico lab in Virginia for further analysis as the agency launched its own probe.

They were found to contain boric acid, which can irritate the skin, cause shortness of breath if inhaled and lead to vomiting if ingested, according to the National Pesticide Information Center.
“ICE is extremely grateful to the FDNY, NYPD, FBI and FPS for their swift and decisive action to ensure the safety and well-being of everyone who was inside 26 Federal Plaza during this incident.” an ICE spokeswoman told The Post in a statement Friday. “The collaborative efforts of our federal and local partners resulted in the building being cleared to open for normal operations today.

“The perpetrator(s) of this appalling crime who sought to terrorize ICE employees will not prevent us from continuing our congressionally mandated mission to preserve public safety and national security by keeping dangerous criminal aliens out of our communities in New York City and off the streets of America. As the investigation remains ongoing, ICE defers any further comment to the FBI.”

Christopher Raia, Assistant Director in Charge of the FBI’s New York office, told reporters during a news conference Thursday night, “Sending threatening letters of this nature, whether real or a hoax, is a crime.

“It diverts valuable, limited law enforcement resources as you’ve all seen tonight.”

The shocking incident, seemingly targeting ICE’s offices, comes two days after a federal judge ordered the agency to improve its immigration holding facility following complaints from jailed migrants over inhumane conditions.

Judge Lewis A. Kaplan ruled Tuesday in a lawsuit filed on behalf of detainees that the immigration agency must limit capacity, keep holding rooms clean, and provide sleeping mats in so-called hold rooms.

The lawsuit alleged dirty, foul-smelling, and overcrowded cells where detainees were denied soap, toothbrushes, and other basic hygiene and menstrual products. 

The Lower Manhattan building has become a hub for arrests as the Trump administration continues its aggressive crackdown on illegal immigration.

ICE facilities and officers have been targeted by violence since the administration ramped up arrests and deportations. 

Last month, 10 members of an armed mob were charged with attempted murder after opening fire on local cops, unarmed federal agents and detention workers at the Prairieland Detention Center near Dallas, Texas. 

One officer was shot in the neck when one suspect fired 20 to 30 rounds after the deranged group set off fireworks at the ICE facility and spray-painted cars parked at the building, according to the criminal complaint.