Authorities say the envelope contained a letter with the word "anthrax" circled with an arrow pointing to the substance.
A clerk who opened the letter and two other people were being decontaminated as a precaution. Federal and local authorities who responded to the Brooklyn courthouse also were conducting field tests on the powder.
It was not immediately known who the letter was addressed to.
About two dozen people were quarantined in the clerk's office for about 45 minutes, but the courthouse wasn't evacuated.