Security is very high on the New York subway
Federal agencies had been warning of the possibility of further terrorist strikes, and cautioned Americans to be extra vigilant.
In the latest security scare, a subway station in Prince George County just outside Washington DC was closed down on Tuesday after a man sprayed a substance into the air and scuffled with police, who then arrested him.
"Tests do not indicate these are any biological agents at all," said Prince Georges County Police Chief John Farrell, after special hazardous-material teams in rubber suits examined the scene.
Some 35 passengers and subway employees complained of nausea, headache and dry throat and were isolated for decontamination, officials said.
The man also dropped a jar of clear liquid, which spilled on the train and platform.
'Perfume or detergent'
Initial analysis of the substance indicated that it was perfume or cleaning detergent.
A public transport spokesman, Ray Feldmann, said the incident occurred when the man police arrested refused to pay for a metro ticket.
When another officer arrived and they attempted to arrest the man, he pulled out a handgun and fired, not hurting anyone.
Police also found a steak knife in his possession, Mr Feldmann said.
Heavy precautions
Meanwhile, the new head of the US Office of Homeland Security, Pennsylvania Governor Tom Ridge, promised "strong precautions," but admitted that America was "not immune" to attack.
US Attorney General John Ashcroft said for his part that extra physical security was in place in and around US nuclear facilities, and at chemical and petrochemical plants.
The enhanced security measures include more intensive air patrols over the US, with Nato sending five Awacs surveillance planes to US air space.
The National Guard is deploying in New York and elsewhere as part of the new security measures.