An internal probe has cleared the FBI agents who obtained a false confession from a one-time suspect in the Sept. 11 attacks. But the findings raise new questions about the reliability of lie-detector tests.

The twin reports released Monday led U.S. District Judge Jed Rakoff to drop his own investigation into the case of Abdallah Higazy.

The Egyptian student was detained after investigators were told an aviation radio capable of communicating with commercial pilots was found in his New York hotel room overlooking the World Trade Center soon after Sept. 11.

He was charged with lying to FBI agents during a polygraph examination in which he supposedly denied owning the radio and then later admitted it was his. But further investigation proved the radio actually belonged to a pilot, and the charges against Higazy were dropped.

Higazy, 31, claimed he confessed only because the polygrapher threatened his relatives in Egypt.

The review by the Justice Department's inspector general found no evidence to substantiate the claim and found "no further action with respect to this matter is warranted."

But the review never explicitly explains why his test results would have indicated knowledge of the attacks, when authorities now say he was not involved.

The reports say Higazy indicated deception when he answered no to the following questions: "Did you take part in those attacks?" and "Were you involved in those attacks?"

The inspector general's reports conclude that "the polygraph was administered appropriately and that the results were properly read to indicate that Higazy was deceptive" in questions about Sept. 11.

The findings imply that Higazy may have confessed to owning the radio under the mistaken impression the admission would get him released from custody.

U.S. Attorney James Comey said Monday he was "very proud of the way our office and the FBI conducted itself in the Higazy case."

In releasing the reports, Rakoff questioned "whether the government's continued reliance on such a doubtful investigatory tool as polygraph testing increases the possibility of false confessions."