The FBI has a new theory on a central mystery of the 2001 anthrax attacks after finding evidence in a pond in Maryland that may suggest how an ingenious criminal could have packed deadly anthrax spores into envelopes without killing or sickening himself.

A piece of equipment and other evidence recovered this northern winter from ice-covered ponds in Frederick Municipal Forest have reinvigorated the 18-month-old case, leading officials to explore a novel theory with shades of science fiction.

Some involved in the case believe the killer may have waded into shallow water to delicately manipulate anthrax bacteria into envelopes, working within a partly submerged airtight chamber. When finished, the killer could have hidden the evidence by dumping contaminated equipment and clothing into the pond. Sydney Morning Herald

The theory strengthens suspicions about former US Army infectious disease researcher Steven Hatfill, who also has diving qualifications.