The substance was later found to be boric acid, a chemical which can irritate the skin, cause shortness of breath if inhaled and lead to vomiting if ingested.

Officials say the substance found Thursday in envelopes in the mailroom of an ICE office at 26 Federal Plaza was not hazardous.

26 Federal Plaza was evacuated after the substance was found inside envelopes in the mailroom.

Kaplan ordered immigration officials to allocate 50 square feet per person – shrinking the largest hold room’s capacity to about 15 people after detainees said 40 or more were being jammed in.

A Brooklyn high school student has been detained by ICE, marking the third arrest of a New York City public school student by immigration authorities in recent months.