A decontamination team at work in Liverpool
A package arrives, or perhaps an envelope, from an unknown sender. Inside is a white powder.
The building is cleared, the police called in, the suspicious substance taken away for testing. It turns out to be harmless - but the fear and uncertainty caused are anything but.
Hoaxers have fanned such fears the world over. Suspect packages have turned up as far afield as Mexico, Malaysia and South Africa in the wake of the outbreak in the United States.
In the UK, the government has acted to try to stop the spate of anthrax hoaxes. From Sunday, anyone found to be behind a chemical, nuclear or biological hoax now faces up to seven years in jail.
Psychologist Paul Mathias, a recently retired detective superintendent with the Metropolitan Police, says hoaxers often need to feel powerful and in control.
"They like to feel that they can control the lives of others, that they can communicate but can't be identified.
"Often they may do it for their own sense of self-esteem. They are keen to identify themselves as somebody who controls lives, which is something they can't do in their everyday lives."
Get 'them' back
With extensive coverage of the events of 11 September and fears of further terror attacks, some may feel that they want to play a part in pushing events along.
"The same thing happened with the IRA troubles," Mr Mathias says. "A number of hoaxers have been unable to offer any explanation other than they thought something should be happening."
Others may make bogus threats for attention, for revenge for some perceived slight, or even for a laugh, says Gerard Bails, a consultant forensic psychologist.
And in some cases, those sending suspect packages will be sympathetic to the terror attackers' cause, Mr Bails says.
"This is an extremely effective terrorism technique because it causes disruption and chaos. It's a way of causing fear without actually fighting your enemy, without putting yourself in danger."
False alarms the world over
In uncertain times, even the slightest perceived threat can cause panic.
On Friday, experts warned that the fear of biological attacks may cause more illness than the weapons themselves. In a letter to the British Medical Journal, researchers in the UK, US, and Australia said fear and anxiety could remain high for years.
Already, there are reports of mass "sociogenic" illness. On 29 September, for instance, paint fumes set off a bioterrorism scare at a school in Washington State, sending 16 students and a teacher to the hospital.
Mr Mathias puts this in part down to our herd mentality.
"At workplaces where some people respond with extreme anxiety, others who might not normally react like that will feel they have to exhibit the same concerns."
Yet for many, the attacks on New York and Washington punctured their sense of safety in the world and left them unsure how to react.
And it is this uncertainty that the hoaxers seek to exploit.