US authorities are looking for a letter addressed to the CIA and believed to contain traces of the poison ricin.
The letter, posted to a CIA office that does not receive mail, has been linked to a man charged with sending a ricin letter to a judge, the FBI said.
The man is also suspected of sending a ricin letter to President Barack Obama.
In separate cases, New York City Mayor Michael Bloomberg and one of his aides, as well as a US senator, were sent ricin letters.
‘Do not open’
On 22 May, Matthew Buquet, 38, was arrested in Washington state and charged with sending one of the letters to federal judge Fred Van Sickle.
Late on Thursday, the FBI in Seattle revealed it had linked ricin-laced letters addressed to a US post office and Mr Obama to Mr Buquet.
A fourth letter also linked to him, sent to the Fairchild Air Force Base near Spokane, is undergoing tests.
The FBI said a fifth letter, addressed to the CIA but to an location that does not receive postal deliveries, had not been located.
“Active monitoring of the mail stream continues in an effort to locate this letter and mitigate any risk its contents might pose,” the FBI said in a statement.
It added in the unlikely event that a member of the public comes across the letter, it should not be opened and the authorities should be informed.
All four letters that have been located were postmarked in Spokane, Washington on 13 May and the addresses were handwritten in red ink.
Mr Buquet’s case represents the third in a recent spate of ricin letter threats in the US aimed at Mr Obama and others.
In a second case, letters containing ricin were sent to Mr Bloomberg and a senior aide at his gun control advocacy group. A “similar” letter addressed to Mr Obama was also intercepted, authorities say.
The mail addressed to Mr Bloomberg and his adviser contained a threat that the sender would “shoot in the face” anyone who tried to take away his or her guns.
Authorities have made no arrests in that case.
In a third case, a Mississippi man has been charged with sending ricin-laced letters to Mr Obama, a judge and a Mississippi senator.
Ricin is a naturally occurring protein found in the castor oil plant that is highly toxic. It is 6,000 times more poisonous than cyanide.
Ricin can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.
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