The letter returned a presumptive positive for cyanide, but it was being tested again to confirm the result, the Secret Service said tonight, confirming a story first reported by The Intercept.
The letter was received Monday at the White House Mail Screening Center, and after initial biological testing came up negative, chemical testing today came up with the positive for cyanide.
The sample was transported to another facility to confirm the results, the Secret Service said.
Sources familiar with the case say told ABC News the substance was inside a plastic Ziploc-like bag inside an envelope.
The sender is someone known to the Secret Service and charges are possible, depending whether the material was indeed hazardous, the sources said.