Former Army scientist Steven Hatfill landed once again at the center of the FBI's 10-month anthrax investigation last week, stirring debate within the agency. Hatfill whom the FBI has called a "person of interest" in the probe, along with about 25 other scientists held a press conference in which he vehemently denied any role in mailing the anthrax-laced letters that killed five last fall. But days later, FBI agents and Princeton police were flashing Hatfill's photo to merchants in Princeton, N.J., where anthrax spores turned up on a mailbox near Princeton University. The sleuthing is controversial among some veteran investigators, who fear that by displaying only Hatfill's picture to locals, rather than a photo lineup of several people, the officials were biasing witnesses, says a source with knowledge of the anthrax case. The concern is that this method could lead to false sightings, or if charges are filed, witness testimony could be thrown out. Nevertheless, investigators are hopeful that the discovery of the "hot box," as the mailbox is called, represents a significant break in the case. The box was one of hundreds that fed into a postal sorting center in Hamilton Township, N.J., where the four anthrax-laden letters were postmarked. The next step is to check police records of traffic and parking tickets issued in the area for any connections to scientists who are on the FBI's list of potential suspects. Authorities will also look for other links to the scientists, such as a hotel stay or meal in the area.