Four suspicious letters that appear to have come from the same person were sent to the White House, the Pentagon campus and Sen. Ted Cruz's campaign headquarters in Houston on Monday and Tuesday, law enforcement officials told NBC News.
An envelope sent to the White House on Monday never entered the building, the Secret Service said. Also Monday, two envelopes suspected to contain ricin, according to initial testing, were detected at a mail processing center on the Pentagon campus, a Pentagon spokesman said. And on Tuesday, two people were taken to a hospital at Cruz's office after being exposed to a "white powdery substance," the Houston Fire Department told NBC DFW.
Officials told NBC News they do not consider any of the letters to be dangerous. They said they have promising leads on where the letters originated. No information on a suspect was released.
The Department of Defense allowed reporters to photograph one of their mail screening rooms. Workers wear full protective suits.
The envelope sent to the White House was addressed to President Donald Trump, the Secret Service said.
The two suspicious envelopes on the Pentagon campus were addressed to U.S. Secretary of Defense James Mattis and U.S. Chief of Naval Operations Adm. John Richardson, a source familiar with the incident told NBC News.
Security officials found a suspicious substance during mail screening, Pentagon spokesman Col. Robert Manning III said Tuesday afternoon.
"On Monday, the Pentagon Force Protection Agency detected a suspicious substance during mail screening at the Pentagon's remote screening facility," Manning said in a statement.
The FBI said tests on the envelopes were being conducted Tuesday.
A federal official told NBC News that a field test indicated the presence of ricin. Those tests can be unreliable, the official said.
All mail received at the facility was quarantined and poses no threat to Pentagon personnel, Manning said.
Ricin is a poison found in castor beans. It can be a mist, a powder or dissolved in water, according to the Centers for Disease Control and Prevention. If inhaled or ingested, it can be deadly.
Stay with NBC for more details on this developing story.