Then the letters went through the postal system, rubbing against other letters with other scents. And finally, the letters were decontaminated using radiation, which might affect the scent.
Two false trails
The use of the dogs in the sniper case raised other doubts for some handlers, although the dogs apparently played no role in the identification of two suspects last week.
A Maryland law enforcement officer involved in the sniper investigation said the dogs from California, given the scent taken from spent shell casings, followed two false trails in Montgomery County.
One led to a house, for which a search warrant was obtained and which turned out not be relevant. The other led to a dog-grooming parlor, the officer said.
Others defended the Californians' work, particularly on the anthrax case. Two people in touch with FBI investigators said the three dogs were given scents taken from three different letters, and all consistently identified Hatfill while ignoring other potential suspects.
`Proud' of colleagues
Harris, the inventor of the Scent Transfer Unit, defended his three California colleagues. "Until these [critics] have tried it and tested it, they shouldn't talk about it. I'm extremely proud of these young people doing this work," said Harris, 74.
Do, the Los Angeles County prosecutor, also said the California bloodhound handlers ran tests in which they had a person touch an envelope, irradiated the envelope in the way the anthrax letters were decontaminated - and found the dogs could still track the person successfully using the scent taken from the irradiated letters.
In any case, Do said, while rules vary from court to court, nowhere can bloodhound evidence alone be used to convict a person of a crime. There has to be other, corroborating evidence, she said.
"The dog is not telling you this person committed the crime," she said. "All the dog is telling you is that the scents match. It's like a fingerprint. The fact that your fingerprint is on the gun does not mean you fired it."