Biohazard sensors showed the presence of small amounts of potentially dangerous tularemia bacteria in the Mall area last weekend as huge crowds assembled there, but health officials said they believed the levels were too low to be a threat.

Health authorities in the Washington area were notified yesterday that the bacteria were found in and near the area between the U.S. Capitol and the Lincoln Memorial, where crowds gathered Saturday for an antiwar rally and a book festival.

The notification, which came from federal health officials, said that after the initial detection, subsequent tests "supported the presence of low levels" of the bacteria. However, officials also said they did not believe the findings posed a health problem.

"We pretty much feel there is no public health threat here," said Von Roebuck, a spokesman for the federal Centers for Disease Control and Prevention, noting that there have been no reports of tularemia, the disease that is caused by the bacteria. "We just wanted to alert the medical community to watch out for cases."

Health officials said the usual incubation period for tularemia is less than a week.

Roebuck said people who were on the Mall but who do not have symptoms need not be concerned.

Symptoms include fever, chills, headache, muscle ache, joint pain, dry cough and conjunctivitis.

Officials said the quantities detected were too small to have been an attack.

In nature, the bacteria are found in rodents and small animals, and "the working hypothesis" is that something in the environment got stirred up, D.C. Public Health Director Gregg A. Pane said.

But he said it was puzzling that the finding was from a day when the Mall was packed with people.

"Why that day? That's what is not explained," Pane said. "It was just this 24-hour period and none since."

At least one official suggested that so many people on the Mall might have triggered the alert, since dry conditions would have made it easier to raise dust.

Tularemia is not spread from person to person. It can be contracted by direct contact with the bacteria that cause it -- by swallowing them or, if they have been suspended in air, through inhalation.

The germ that causes tularemia is considered a biohazard because it is highly infectious and was tested in the 1960s by the United States as a biological weapon. The disease is treatable with antibiotics but, if left untreated, can be fatal.

The country has spent more than $200 million to install the sensor system known as BioWatch in more than 30 U.S. cities. Samples from sensors are collected daily to check for pathogens such as those that cause anthrax, smallpox or plague.

More than a half-dozen sensors operating from 10 a.m. Saturday to 10 a.m. Sunday -- at sites including the Lincoln Memorial, Fort McNair and Judiciary Square -- detected the bacteria, Pane said he was told.

He said the CDC expected to notify hospitals nationwide as a precaution because so many people came from out of town to the Mall last weekend.

Similarly, he said, he expected area health officials to watch for symptoms into next week.

Authorities recommend that people who visited the Mall between 10 a.m. Sept. 24 and 10 a.m. Sept. 25 should see a physician if they experience symptoms.

Staff writers Spencer S. Hsu, Daniel K. Eggen and Rick Weiss contributed to this report.