WASHINGTON - The top FBI official in charge of the investigation into the deadly anthrax attacks has left the case, NBC News has learned. Richard "Rick" Lambert had been the inspector of the so-called AMERITHRAX case since September 2002, and had run every aspect of the five-year-old investigation. Just last month, he was transferred to the Knoxville, Tenn., field office of the FBI as its special agent in charge, according to the FBI.

Lambert was the public face of the case, and his transfer is sure to fuel speculation that the massive investigation has stalled. No one has been arrested, five years after the first anthrax-laden envelopes were mailed from a New Jersey post office, and officials familiar with the case tell NBC News that no criminal charges are expected to be filed anytime soon.

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Lambert was not available for comment Monday, but an FBI official tells NBC it's unfair to read too much into his transfer. The case "is not stuck in the mud," the FBI official says, adding that it's standard practice in the FBI to rotate senior officials on and off major cases. (A series of senior FBI agents ran other high-profile investigations, including the hunts for serial bomber Eric Rudolph and the Unabomber, "Ted" Kaczynski.)

Later Monday, the FBI plans to issue a press statement stating that hard work on the case continues. The FBI will say that it is one of the largest, most complex cases in its history and the the FBI is "committed to solving the case."

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