The laboratory that initially reported that anthrax (search) was present in a Pentagon (search) mailroom said Wednesday that an extensive review produced no evidence the lab was to blame for what authorities later concluded was a false alarm.

"Unfortunately, the source of the positive sample may never be known with certainty," said a statement from Dr. Robert Harris, chief operating officer of Commonwealth Biotechnology Inc. (search) in Richmond, Va., which did the initial testing.

He added that the lab stands behind its initial results, despite official conclusions to the contrary.

The positive report last week contributed to officials closing two mailrooms that serve the Pentagon, plus a U.S. Postal Service facility that handles government mail. Nearly 900 workers were given antibiotics as a precaution, in case they were exposed to anthrax.

After further testing, officials concluded there was no anthrax present.

The working theory was that Commonwealth Biotechnology employees contaminated the Pentagon sample with anthrax kept on hand for comparison purposes, a Homeland Security Department official, who spoke on condition of anonymity, said last week.

The initial sample was later delivered to a Defense Department laboratory at Fort Detrick, Md. (search), which confirmed the presence of anthrax.

The Richmond lab, which conducts daily tests of swabs from the Pentagon mailroom, said Wednesday that after a thorough review it concluded there was no evidence of surface or airborne contamination. However, the review recommended "minor modifications" to procedures used for analysis.

Separately Wednesday, local authorities said they were launching a formal "after-action review" to look at how they handled the scare.

"Before we can draw accurate conclusions about what went well and areas needing improvement we need facts," said a statement from Virginia Gov. Mark Warner. District of Columbia and Maryland officials are also participating in the review.