WASHINGTON?Government officials said Sunday that no new cases of anthrax contamination had been found, but they are continuing to take precautions in case other tainted letters are in the mail system.

Thousands of postal workers and others who dealt with large amounts of mail were being urged to take preventive antibiotics.

“There may be other letters that are stuck in the system,” White House Chief of Staff Andrew Card said on “Fox News Sunday.” “We’re asking people to be very careful.”

Deputy Postmaster General John M. Nolan said on CBS’s “Face the Nation” that there are many suppositions among investigators about more letters, “but I don’t have any way of knowing.”

“There is no evidence of new anthrax in the system anywhere,” he said.

Despite the strain on the system, postal vice president Deborah Willhite vowed that the mail will go through.

“The Postal Service will do whatever it takes to continue to serve the American people,” she said.