The postal facilities are being checked rigorously
Their bodies are being tested for anthrax symptoms amid circumstances described by health officials as "highly suspicious".
The revelation came as US authorities confirmed that a second postal worker in the city had tested positive for anthrax infection, bringing the number of cases of infection nationwide to 10.
The new victim, who has not been named, has the more serious inhaled form of the disease.
The two dead postal workers handled mail for the US Congress in Washington DC.
Of the cases of anthrax that have now been identified in the US, six are skin anthrax and four are inhaled anthrax.
One man, an employee of a tabloid newspaper in Florida, has died as a result of anthrax inhalation.
The head of Washington Health Department, Dr Ivan Walks, said that the two dead postal workers had both worked at the Brentwood facility in Washington - the workplace of the two people who are confirmed cases.
Dr Walks called on their colleagues to report in for tests.
"Anyone who was working in that back postal area during the last 11 days, you must today immediately come here... to receive medication and to be evaluated," he said.
US law enforcement officials have said there is no evidence linking the anthrax cases with foreign terrorists.
But with nerves on edge since the 11 September terror attacks on New York and Washington, the two threats are being increasingly linked.
In other developments:
Both the Brentwood postal facility and a postal centre at Washington-Baltimore International Airport have been closed indefinitely for environmental tests, and the 2,200 staff who work at them are being tested in hospital.
Anthrax fears came to the fore when infected packages began arriving at media companies and government buildings in Florida, New York and Washington.
The US Capitol building, home to both houses of Congress, reopened on Monday after closing last week following the discovery of anthrax-infected post there.
The US House of Representatives and Senate will reconvene for debate on Tuesday, although congressional office buildings near the Capitol will remain closed until health officials deem them anthrax-free.
Experts have been inspecting the US Capitol and three Senate office buildings, in addition to the underground system linking the buildings, since the discovery of anthrax-infected post last week.
Anthrax spores were found in a mail processing room in the House of Representatives after a letter sent to Senate leader Tom Daschle was found to contain the bacteria.
One confirmed anthrax death
The only person confirmed to have to died from anthrax during the scare - a Florida newspaper journalist - had the inhaled form of the disease.
US Homeland Security Chief Tom Ridge has said the strain of anthrax used in the attacks on US media companies and the Senate appeared to be the same.
The US is offering $1m for information on who is behind the anthrax attacks, FBI director Robert Mueller said.
The US Postal Service is sending cards to every home and business - 147 million addresses - telling people how to deal with suspicious post.