Officials say there are no signs anyone got sick from anthrax after a lab safety problem that may have put dozens of workers at risk at the Centers for Disease Control and Prevention.
Earlier this month, the CDC said dozens of scientists and other workers at three labs in Atlanta may have been accidentally exposed to anthrax.
But this week, CDC officials said anthrax spores have not been found on surfaces in the labs and it's not clear that any dangerous anthrax was actually released.
About 60 workers were offered antibiotics and vaccinations as precautionary treatment. The CDC says about half can now stop taking the medication.
A CDC internal report on what went wrong is expected later this week. Earlier, the federal agency said anthrax samples used in research hadn't been sterilized as expected.
The news of the anthrax exposure first emerged on June 19 andthe agency announced it was taking action to protect employees.
Bacillus anthracis, the bacteria that causes an anthrax infection, is found naturally in soil and can be picked up by both domestic and wild animals.
Though rare, anthrax infections can occur in humans when spores enter the body through inhalation, food or water consumption. Once spores are activated they release toxins that can have a systemic impact on the body. If caught early the infection can be successfully treated with with antibiotics, such as ciprofloxacin or doxycycline.