Anthrax tests at two US defence department mail sorting offices have proved negative, US officials say.
At least five people died in anthrax attacks in 2001
The scare had been set off a day earlier when initial tests detected signs of the deadly bacteria.
The two centres - and a mail handling facility in Washington - were shut and some 200 staff were given antibiotics.
AP reported that a sample of anthrax, which is kept for comparison purposes, may have got mixed up with the samples taken from the mail offices.
"We have no evidence that there was anthrax material in the mail" said William Winkenwerder, assistant secretary of defence for health affairs.
"We had some preliminary results that were positive, but subsequent additional tests have determined that the sample that we had was in fact negative."
However, he added that some additional tests were still incomplete.
Mr Winkenwerder said the two Pentagon facilities could reopen on Wednesday morning.
A spate of anthrax attacks in October 2001 remain to be solved.
At least five people died - including two Washington postal workers - and 17 were taken seriously ill within weeks of the attacks.