Four years after anthrax attacks killed five Americans, one of the most exhaustive investigations in FBI history has produced no arrests – and is showing signs of growing cold. In the past year the number of FBI agents working on the case has dropped from 31 to 21, and the number of postal inspectors has fallen from 13 to 9. FBI officials say investigators are still working diligently to find who was responsible for the anthrax-laced mailings, the Washington Post reports.The anthrax attacks killed five people in Florida, New York, Connecticut and Washington, D.C., sickened 17 and led to the temporary shutdown of the House, Senate and Supreme Court buildings.