Both letters were signed, "I am KC and I approved this message," according to NBC News.
The two letters were transferred to a secondary testing facility in Maryland, according to the law enforcement officials, who spoke on the condition of anonymity because the investigation was ongoing.
A glimpse of the investigation came outside a low-slung, white-and-green postal warehouse north of FedEx Field in Landover, Md., where county fire crews helped emergency workers suit up in white-and-yellow hazmat suits before trudging inside a yellow tent. Two black SUVs and a car bearing the insignia of the FBI Joint Terrorism Task Force sat behind the building.
In Phoenix, a suspicious letter was discovered Wednesday at the office of Sen. Jeff Flake, R-Ariz. By afternoon, Flake said in a statement that tests on the letter were negative for ricin.
Sen. Carl Levin, D-Mich., also said in a statement that a staffer received "a suspicious-looking letter" Wednesday morning at the senator's office in Saginaw, Mich.
"The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating," Levin said. "We do not know yet if the mail presented a threat."