The FBI is trying to keep secret technical details of a keystroke-logging program it used to catch a Mob loan shark.

A mandate from a New Jersey judge calling for a report "detailing how the key logger device functions", by 31 August, has been claimed by the US government to be so sensitive that if revealed, "national security" could be at risk.

The FBI is now seeking to invoke the Classified Information Procedures Act (CIPA), so that details are not supplied.

The technology, which captures the keystrokes made on a computer and can be used to discover encrypted passwords, is at the centre of the case of the United States of America v. Nicodemo Scarfo, the alleged brains behind a loan shark operation based in New Jersey.

Scarfo has been charged with racketeering, illegal gambling and loan sharking as a member of what the Department of Justice calls "a Mafia crime family".

Scarfo's father is jailed Mob boss Nicodemo "Little Nicky" Scarfo Sr.

According to court documents, defendant Scarfo has requested discovery concerning the underlying functionality of the FBI's Key Logger System. The warrant required that the key logger device not be used to monitor Scarfo's internet traffic since it requires a warrant equivalent to one for a wire tap.

Without the details of the system's operation, the defence argued that it could not determine if the technology violated the warrant.

The Government has maintained in its prior two submissions, dated 17 July and 3 August 2001, that the defendants are not entitled to this information since it is beyond the purview of Federal Criminology and is privileged.

And the prosecutors say if details of the key logger system are made public, it will be useless in further investigations and will jeopardise current ones.

David Sobel, general counsel for the Electronic Privacy Information Center (EPIC) in Washington, said: "CIPA was never intended for criminal proceedings, but only for unusual national security cases."

He said the challenge is to ensure these new techniques are used in a constitutional way.