A suspicious envelope that was addressed to President Donald Trump was intercepted by the Secret Service on Monday, US authorities say.
The envelope did not enter the White House, according to the authorities.
Two other packages suspected of containing the deadly poison ricin were found at a Pentagon mail screening facility and sent for FBI analysis.
Those envelopes - one of which was addressed to Defence Secretary James Mattis - were also found on Monday.
Mr Mattis was away in Paris at the time.
The screening area is not inside the main building, which contains offices of high-ranking military officials.
The facility is now under quarantine, defence officials said on Tuesday.
The Secret Service said in a statement that the envelope addressed to the president "was not received at the White House, nor did it ever enter the White House".
It said it was "working jointly with our law enforcement partners to fully investigate this matter".
The two packages sent to the Department of Defence were detected by the Pentagon Force Protection Agency, according to the Pentagon.
All mail delivered on Monday is currently under quarantine, and "poses no threat to Pentagon personnel", a spokesman said.
The other envelope was addressed to Chief of Naval Operations John Richardson.
Up to 25,000 people go to work at the Pentagon every day.
Ricin is made from the leftover by-products of castor oil.
According to the US Centers for Disease Control and Prevention: "It would take a deliberate act to make ricin and use it to poison people."
The federal agency said it can be manufactured into a weapon in the form of a powder, mist or pellet.
Two people were taken to the hospital after apparently being exposed to a white powdery substance in an office building at 3200 SW Fwy. The 9th floor of the Phoenix Tower has been evacuated as HFD HazMat is responding to the scene working to determine the nature of the substance.— Houston Fire Dept (@HoustonFire) October 2, 2018
Also on Tuesday, two people at Republican Senator Ted Cruz's office in Houston, Texas, were taken to hospital after coming into contact with a "white powdery substance", officials say.
The Houston Fire Department tweeted that a hazardous materials crew is on scene at the evacuated office building.
Houston Police said they were taking over the investigation into the letter, which comes as Mr Cruz is locked in a fierce election battle against Democrat Beto O'Rourke.
A spokeswoman for Mr Cruz told the Associated Press that no staff were affected by the incident.