WASHINGTON, Sept. 16 (Xinhuanet) -- Four years after the deadly 2001 anthrax attacks, the US Federal Bureau of Investigation (FBI) has made little progress in one of its most exhaustive investigations, The Washington Post reported Friday.

    The investigation has yielded no arrests and is showing signs of growing cold as the number of FBI agents on the case has been sharply reduced from 31 to 21, and the number of postal inspectorshas fallen from 13 to nine, the report said.

    The agents and inspectors have pursued leads on four continents, conducted more than 8,000 interviews and carried out dozens of searches of houses,laboratories and other locations, it said.

    They traveled to Afghanistan twice in the past 16 months to follow up on tips that proved fruitless, the report quoted law enforcement sources as saying.

    Investigators were still working to find who was responsible for the anthrax-bacteria-laced mailings, which occurred just afterthe Sept.11 attacks, killed five people, sickened 17 others and led to a temporary shutdown of the House, Senate and Supreme Courtbuildings and numerous postal facilities, FBI officials told the newspaper.

    The investigation has been so expansive that authorities now are in the process of taking inventory, according to the Post report.

    The FBI and postal inspectors have spent months piecing together a voluminous internal report that will review the scope of the investigation and explore issues including what has been a prevailing theory: the culprit is a US scientist who had access tothe high-grade anthrax and the knowledge of how to physically manipulate it and use it as a weapon, said the report.

    Authorities hope the report, which is to be completed soon and forwarded to FBI Director Robert S. Mueller III, will provide a concise road map of the probe and help determine its future direction, it said. Enditem