A relatively rare biological agent has been detected in air filters serving Washington D.C., in recent weeks, ABC News has learned -- but current evidence does not show any indication whatsoever of terrorism.
The federal government found six air filters around the nation's capital checked on Sept. 24 and 25 contained "trace amounts" of tularemia, a type of bacteria.
The tests were conducted as part of routine homeland security procedures, in which selected air filters from around the nation's capital are checked for chemical and biological agents.
Subsequent tests by the Centers for Disease Control confirmed low levels of the bacteria.
Tularemia causes flu-like symptoms, including fever, chills and muscle pain, but it is not generally considered life threatening. People who inhale the bacteria, however, can experience severe respiratory illness, including pneumonia and systemic infection if they are not treated, according to the CDC.
About 200 human cases of the disease are reported in the United States each year. The disease, also known as "rabbit fever," can be treated with several common antibiotics.
Tularemia is a naturally occurring disease typically transmitted by rodents and dead carcasses, and has been reported in the past in all U.S. states except Hawaii. Homeland security and FBI officials say no human illnesses have been reported, and that the health risk is low.
No additional traces have been detected since Sept. 25. One law enforcement official said drought conditions in Washington and heavy pedestrian traffic may have sent particles airborne. No biodefense sensors have gone off.
There are concerns about tularemia being used as a bioweapon because it is highly infectious, and a small amount of bacteria can cause disease. It can remain alive for weeks in water and soil.
ABC News correspondents Pierre Thomas and Jack Date filed this story.