Buried in FBI laboratory reports about the anthrax mail attacks that killed five people in 2001 are data suggesting that a chemical may have been added to try to heighten the powder's potency, a move that some experts say exceeded the expertise of the presumed killer. The lab data, contained in more than 9,000 pages of files that emerged a year after the Justice Department closed its inquiry and condemned the late Army microbiologist Bruce Ivins as the perpetrator, shows unusual levels of silicon and tin in anthrax powder from two of the five letters. Those elements are found in compounds that could be used to weaponize the anthrax, enabling the lethal spores to float easily so they could be readily inhaled by the intended victims, scientists say. The existence of the silicon-tin chemical signature offered investigators the possibility of tracing purchases of the more than 100 such chemical products available before the attacks, which might have produced hard evidence against Ivins or led the agency to the real culprit. But the FBI lab reports released in late February give no hint that bureau agents tried to find the buyers of additives such as tin-catalyzed silicone polymers. The apparent failure of the FBI to pursue this avenue of investigation raises the ominous possibility that the killer is still on the loose.