"No additional information will be released to the public today," said Frank Harrill, the FBI agent in charge of the Spokane office.
Agency spokeswoman Ayn Sandalo Dietrich added legal documents in the case also might be sealed. Their comments come after law enforcement officers raided a Spokane apartment Saturday and witnesses said they escorted a man from the building.
The letters were postmarked last Tuesday in Spokane and addressed to the downtown post office and the adjacent federal building. They were intercepted by the Postal Service, and no one was injured.
Investigators in hazardous materials suits spent most of Saturday executing a search warrant at the three-story apartment building. Authorities said there was no public health risk.
Everything seemed back to normal at The Osmun apartments Monday morning, with no signs of Saturday's raid. There was no answer after a knock on the door of the apartment that was raided.
Witnesses have said a man was escorted from the building by law enforcement officers Saturday morning. Sandalo Dietrich declined to say if anyone was being questioned in connection with the letters.
"There have been no arrests and no charges filed," she said Monday.
On Saturday, Sandalo Dietrich said the FBI was "not actively looking for a subject."
Authorities have not released a potential motive. They also have not said whether the letters targeted anyone in particular.
Ricin is a highly toxic substance made from castor beans. As little as 500 micrograms, the size of the head of a pin, can kill an adult if inhaled or ingested.
There have been no reports of illness connected to the letters. The Postal Service has said the ricin was in a crude form that did not pose a health risk to its employees.
The Spokane investigation comes a month after letters containing ricin were addressed to President Barack Obama, a U.S. senator and a Mississippi judge. A Mississippi man has been arrested in that case.
Ricin has been called the poor man's bioterrorism because the seeds are easy to obtain and the extraction process is relatively simple. Ricin is much more toxic than cyanide, and is made from the waste meal left after processing castor beans for castor oil. It can be in the form of a powder, a mist, or a pellet, or it can be dissolved in water.
In one of the most famous assassinations of the Cold War, Bulgarian secret police in 1978 used a tiny pellet of ricin, fired from a specially designed umbrella, to kill dissident Georgi Markov on a street in London.
This is not Spokane's first brush with the poison. In 2002, Kenneth R. Olsen of Spokane was arrested for possession of ricin in his office cubicle at a high-tech company.
A co-worker tipped off the FBI about the software engineer after discovering documents about poisons and bomb-making that Olsen had printed from his computer. Olsen insisted his research was for a Boy Scout project.
He was sentenced to almost 14 years in federal prison for possessing a chemical weapon and possessing a biological weapon.