Nassau Police spokesman Lt. Kevin Smith says a small container inside the woman's car included a notation that it was sodium cyanide. He says tests showed it wasn't dangerous. The substance was being sent to an FBI lab for further analysis.
An area around the Bellmore Post Office was cordoned off after 10 a.m. Friday after federal marshals arrested the suspect, an unidentified woman in her 40s. Authorities say she was wanted on out-of-state arrest warrants for identity theft and other charges.
The suspect, along with three marshals and a Nassau ambulance technician were sent to a hospital as a precaution.