Disclosure of what the FBI knows about the deadly 2001 anthrax (search) attacks could enable terrorists to engineer biological weapons to escape detection, the FBI (search) says in documents filed in response to a lawsuit by a scientist labeled a "person of interest" in the case.

Citing the criminal investigation and national security concerns, the Justice Department is trying to persuade a federal judge to delay the lawsuit filed by Dr. Stephen J. Hatfill (search), who contends the government invaded his privacy and ruined his reputation by leaking information to the media implicating him in the attacks.

Hatfill has denied any role in the attacks and his lawsuit seeks to clear his name and recover unspecified monetary damages.

Richard L. Lambert, the FBI inspector in charge of what is being called the "Amerithrax" investigation, says in a court document that Hatfill's lawsuit could jeopardize the probe and expose national secrets related to U.S. bioweapons defense measures.

"In the hands of those hostile to the U.S., this valuable intelligence could aid state sponsors of terrorism or terrorist organizations in their efforts to genetically engineer or alter their anthrax bioweapons to 'spoof' or escape detection," Lambert said.

Disclosure also would make public the vulnerabilities and capabilities of U.S. government installations to bioweapons attacks and expose sensitive intelligence collection sources and methods, Lambert said.

There is no proven link between terrorist groups and the October 2001 anthrax attacks that killed five people and sickened 17 others. The government has, however, repeatedly warned of Al Qaeda's interest in using anthrax or other chemical and biological weapons to mount attacks.

Hatfill's attorneys filed a reply Tuesday opposing any delay in his lawsuit and calling it a "monumental irony" that the government was now stressing the need for secrecy in the case.

"Having engaged in a campaign to smear Dr. Hatfill by disclosing confidential information from their investigative files, the government now requests that the court grant an indefinite delay of any inquiry into their illegal conduct," says the brief filed by attorney Thomas Connolly and others.

In the FBI document, filed Nov. 21 in U.S. District Court in Washington, Lambert calls the anthrax probe "unprecedented in the FBI's 95-year history" because of its scope and complexity. In all, the investigation has consumed some 231,000 agent hours, he said.

Lambert described the investigation as "active and ongoing" and said agents' work is divided between checking into individuals who could be linked to the attacks and an intensive scientific effort to determine how the spores themselves were made using "cutting-edge forensic techniques and analysis."

The court papers stop short of confirming that Hatfill is among those being investigated.

Hatfill was labeled a "person of interest" in the probe in August 2002 by Attorney General John Ashcroft and says in his lawsuit that FBI agents have had him under surveillance around the clock.

That surveillance -- which once led agents in a vehicle to run over Hatfill's foot on a Washington street -- has dropped off in recent weeks, according to one person close to Hatfill and two federal law enforcement officials who spoke on condition of anonymity. The officials, however, cautioned against drawing the conclusion that Hatfill no longer was of interest to investigators.

Lambert said in the court document that Hatfill's lawsuit could force the FBI to divulge its "interest in specific individuals," who could then destroy or hide evidence, flee the country, intimidate witnesses or make up alibis. None of these individuals are identified.

The Justice Department is seeking to delay Hatfill's case until a decision is made on a forthcoming government attempt to dismiss the lawsuit entirely.

Hatfill's lawsuit is seeking unspecified monetary damages from Ashcroft, the FBI and Justice Department and other current and former officials. His lawyers contend that the government linked him to the attacks to make it seem that the investigation was making progress.

Hatfill once worked as a researcher at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick in Frederick, Md. Hatfill says he never worked with infectious diseases such as anthrax, however.