In the last two fiscal years, records show, U.S. postal inspectors responded to more than 5,800 reports of letters and packages containing suspicious substances. Only a few dozen cases have resulted in arrests.
"We try to use common sense," said Peter Rendina, spokesman for the U.S. Postal Inspection Service. "We know there are cases where Grandma is sending her favorite muffin recipe and doesn't mean to be a threat."
Scientists disagree over whether the nation is more vulnerable to an anthrax attack today than it was in 2001. (The FBI blames that attack on Bruce E. Ivins, an anthrax researcher at a federal biodefense facility who committed suicide in July.)
The U.S. Postal Service in 2003 installed devices to check for airborne pathogens or poisons at the nation's 271 mail processing and distribution centers. They have yet to detect a threat, Rendina said.
But the boom in biodefense spending carries a danger. Some experts fear that a tenfold increase in laboratories authorized to work with dangerous bioagents increases the risk of leaks. More than 7,200 scientists now are approved to work with anthrax, far more than in the past, creating security risks.
"I think all our screaming about bioterrorism has been counterproductive," said Milton Leitenberg, a University of Maryland scholar who has written extensively about biological weapons. "It's a hard balance to strike."
Boston police felt the same way after the incident at Symphony Hall last month.
An address label on the DHL tube led detectives to a local man, who said he had tossed it in a Dumpster. Police never figured out who picked it up and wrote "Anthrax Beware" on it, or why.
"Happily, there was nothing to it," said Jill McLaughlin, a police spokeswoman. "We've got enough problems without an anthrax scare."