WASHINGTON -- Fire engines. Ambulances. Cropdusters. As raw intelligence comes in about possible terrorist targets, the FBI is sending out alerts across the nation.
It's a tricky balancing act for the government to keep Americans alert without causing panic. And more often than not, the alerts are pulled back hours or days later when investigators conclude the original leads are not credible.
For some citizens, the ebb and flow of these warnings can be emotionally exhausting. But law enforcement believes it has little alternative -- in the midst of an anti-terrorist campaign, authorities would rather be safe than sorry.
"In abundance of caution, we are sending out this information based on information that is developed through investigation no matter the reliability of the source," FBI spokesman Bill Carter said Wednesday.
"In short, if we get information, this information is going out, and later when it is determined the information is not credible, it is withdrawn," he said.
There have been numerous examples since last Tuesday's twin attacks at the World Trade Center and the Pentagon.
On Friday, the cities of Atlanta and Richmond, Va., were warned by the FBI that they might be possible targets of would-be terrorists, based on information from a single witness the FBI interviewed.
When some information from a witness who knew the hijackers checked out, the FBI issued the warnings. Security was markedly stepped up at the Federal Reserve Banks in the two cities.
But the cities were waved off the warning a few hours later when the witness performed badly on a lie detector test, raising questions about the credibility of his information.
Atlanta Mayor Bill Campbell took the whole incident in stride. Of course his city, a major Southeast hub, made sense as a target. And if the threat became serious, the public would have known right away, he said.
"The public would be our greatest weapon," the mayor said.
Around the same time, Washington police received an all points bulletin to watch for a silver van that might be of interest. City police were on the lookout for it.
Earlier this week, the FBI issued a warning to fire departments nationwide that ambulances and fire trucks should be guarded against hijackings or theft. It, too, was eventually rescinded.
"Our deputy chief sent out a notice on Monday to all the departments that says be aware of your surroundings, station apparatus, and emergency scene," said Neil Heesacker, a spokesman for the Portland, Ore., fire department.
"Also all uniforms should be accounted for, but this really isn't new for us. All these precautions are standard procedure for us," he said.
Long after commercial jetliners were allowed to resume flight, agricultural cropdusters were still grounded by the FAA out of concern they could be terrorist targets.
Dennis Gardisser, an agricultural engineer with the University of Arkansas Cooperative Extension Service, said officials told him that there was a fear the small planes used for spraying crops could be hijacked and used to disperse biological weapons.
The farm planes -- usually one seaters -- had been allowed to resume flight last Thursday, but then were grounded anew last Sunday. The government gave final clearance to resume flights Monday after an intensive lobbying effort by farm states.
"They are allowed to fly but they have to stay out of controlled air space," FAA spokeswoman Kathleen Bergen said.
The groundings were of grave concern to some farm states.
"We've got crops in critical condition that need to be treated. Farmers may lose their crops," said Louisiana Agriculture Commissioner Bob Odom.