Shortly before midnight Eastern Time, Capitol Police Chief Terry Gainer and Senate Majority Leader Bill Frist, R-Tenn., appeared before reporters to outline the discovery.
Gainer said that police had been notified at approximately 3 p.m. EST that a white powder had been discovered in a fourth floor office used for sorting mail to the majority leader and others.
A congressional source said the powder was found in an envelope in Frist's office suite. The Dirksen building is one of three structures occupied by senators and their staffs.
Although Gainer said the material had been found on the fourth floor, he said it was unclear what envelope or package may have contained the powder.
One initial test conducted at the scene came back positive for the presence of ricin, a toxin made from the castor bean. Gainer said police initially believed the first result was a so-called "false positive" and tested the material again.
Although the second test was reportedly negative, police removed a sample of the powder for a more detailed and "professional" round of testing. Two of those three tests also indicated the powder was ricin.
Officials said they were awaiting a final, more definitive test conducted at Fort Detrick in Maryland to confirm the presence of ricin, which is twice as poisonous as snake venom.
"At the moment we're in a wait-and-see position from an analytical point of view in what next steps we may take," he said.
Frist stressed that it appeared that none of the 16 people working near the powder had become ill.
"Nobody is sick, we don't expect anybody to get sick," said Frist, who worked as a surgeon before his election to the Senate.
Frist added that the symptoms of ricin poisoning "appear in the first four to eight hours and we are beyond the eight hours."
Gainer and Frist said the employees were undergoing decontamination procedures.
"It has been a trying day," Frist said, adding that a decision about closure of the Dirksen building or offices within the building would be made Tuesday morning.
-- Compiled from wire reports and other media sources