Eleven post offices remained closed this morning in Washington, D.C. after a routine air sample tested positive for anthrax, news reports said.

The air sample showed less than 1% of what it would take to be infectious, a Navy spokesman told the New York Times.

The facilities were closed out of caution while testing continued.

Five workers were given Cipro, an antibiotic, as a precaution against the infectious disease. Anthrax can infect humans through the air or through the skin.

A month after the Sept. 11 attacks, anthrax-laced letters began appearing at post offices and media outlets. Five people died in incidents at that time.

Eleven post offices remained closed Friday in Washington, D.C. after a routine air sample tested positive for anthrax.

The air sample showed less than 1% of what it would take to be infectious, a Navy spokesman told the New York Times, news reports said.

The facilities were closed out of caution while testing continued.

Five workers were given Cipro, an antibiotic, as a precaution against the infectious disease. Anthrax can infect humans through the air or through the skin.

A month after the Sept. 11, 2001 attacks, anthrax-laced letters began appearing at post offices and media outlets. Five people died in incidents at that time.