A letter sent to the White House tested a presumptive positive for cyanide, but was being tested again to confirm, the Secret Service said Tuesday. The envelope was discovered at the White House Mail Screening Center and at first came back with a negative result in biological testing. When tested for chemicals on Tuesday, the results came up positive for cyanide. The substance was reportedly inside a plastic bag in an envelope, sources familiar with the incident told ABC News. The sender is known to the Secret Service and will face potential charges pending the result of the second test, according to sources.