Despite a massive FBI investigation, those responsible for the anthrax attacks on the United States in October 2001 still have to be brought to justice.
The US TV crime show America's Most Wanted still features the anthrax letters, but the special reward of up to $2.5m for information leading to the arrest and conviction of the perpetrators remains unclaimed.
This is surprising because it was a crime that gripped the entire nation.
"We had people in Montana bringing powdered hotdog buns to their state public health laboratories because they were afraid that white powder - which the day before had been flour - was now suddenly anthrax," Dr Elin Gursky from the Anser Institute for Homeland Security.
Five people died in the anthrax attacks
She has just completed a study into the attacks and found the authorities could barely cope.
"It was not only the cases of illness but the fact that it was our mail system, which is pervasive in our offices and our homes, that was used against us," she told the BBC.
Within a few weeks, five people were dead and 17 had been taken seriously ill.
The FBI now believes only four letters were sent - addressed to the New York Post, TV channel NBC, Democrat Senators Tom Daschle and Patrick Leahy.
They were laced with spores of a highly virulent form of the anthrax bacterium - known as the Ames strain.
The investigation has been termed Amerithrax, but the FBI refuses to discuss its progress.
Detectives are taking a particular interest in an area surrounding the US army biodefence research centre at Fort Detrick near Washington DC.
Reports claim divers searching ponds nearby found vials and an airtight container in which the letters could have been sealed.
A germ warfare expert and former US army scientist, Steven Hatfill, has been interviewed several times and is followed on a daily basis by a convoy of FBI personnel.
He maintains his innocence and has yet to be charged with anything.
The former head of the Fort Detrick research programme and now vice-president of the Southern Research Institute, Dr David Franz, urged investigators to keep an open mind.
"The individual or individuals who prepared the formulation were experienced, they need not have special degrees and they were good in the laboratory. It could be a laboratory the size of your kitchen not requiring a load of equipment," he says.
"This expertise would be found in many countries in the world, including the US and the UK."
In her report for the Anser Institute, Dr Gursky warned that if there was another, even small, bioterror attack on the United States, "public health resources are barely adequate".
"Biological warfare will be a grave concern in the next few decades and deserves strong attention in terms of our preparedness," she says.
In the meantime, the person (or people) behind the attacks of autumn 2001 is still at large.
Even if they do discover who did it, the question is, could it happen again? And the answer is almost certainly yes.