The federal government is threatening an official at a Brooklyn jail with imprisonment if he gives legal assistance to several Muslim men who are suing over their detention following the September 11, 2001, terrorist attacks. The guard who received the unusual warning, Steven Barrere, is currently a defendant in a prison abuse suit brought by six Muslim men and a Hindu once held at the federal Metropolitan Detention Center. The men, since deported, are suing prison guards for allegedly beating them up, as well as Attorney General Ashcroft and other top officials for allegedly orchestrating their harsh treatment.

Mr. Barrere, a mid-level officer at the MDC who is accused of pushing the detainees and tripping them while they were in shackles, is in settlement discussions. In return for the suit’s dismissal, Mr. Barrere would “discuss what he observed concerning the plaintiffs,” his lawyer, James Ryan, wrote to the court. That offer drew a stern warning from the government. Depending on its terms, a settlement could prompt the government to file charges against Mr. Barrere, a Justice Department attorney, Stephen Handler, wrote in a letter last week.

A federal law bars federal employees from providing certain assistance in lawsuits against the federal government, although the law is not commonly invoked as an obstacle to settlements.

“The letter did seem a sort of ‘watch your back’ if you cooperate with the other side,” an attorney for the former detainees, Matthew Strugar, said in reference to the government’s position.

The government appears concerned that Mr. Barrere is offering more than just testimony.

“Although you did not provide the details of the offer, you told me that Mr. Barrere is near bankruptcy and is willing to become a consultant for the plaintiffs,” Mr. Handler wrote in a letter to Mr. Ryan.

A spokesman for the Justice Department said the warning was necessary because of ambiguities in the terms of the proposed settlement.

“Because it was and remains unclear as to what being a ‘consultant’ entailed, we sent the letter to apprise counsel of the potential for violating the referenced statute which prohibits providing ‘assistance in the prosecution of’ any claim against the United States,” the spokesman, Charles Miller, wrote in an e-mail message.

Mr. Barrere, who has denied the allegations in the suit, appears ready to settle because of financial pressure. The government is not paying for Mr. Barrere’s defense, the cost of which has left him in “dire financial condition,” Mr. Ryan wrote.

Messrs. Barrere and Ryan did not return calls for comment.