Meeks's office immediately called the Capitol Police when the substance was found.
Lafargue said the powder was found sometime around noon.
This incident follows multiple mailings of ricin, a deadly poison made from castor beans, to a number of elected officials, including Sen. Roger Wicker (R-Miss.) and President Obama.
Since the ricin-laced letters were discovered, the FBI has been investigating.
Over the weekend law enforcement charged James Everett Dutschke with sending the letters. Dutschke was arrested Saturday by federal marshals.