NEW YORK (AP) -- The government's jailing of material witnesses for a grand jury investigation of the Sept. 11 terrorist attacks is unconstitutional, a federal judge ruled Tuesday in throwing out a perjury case against a Jordanian college student.

The ruling, if upheld, could have far-reaching implications for the government's crackdown on terrorism.

Dozens of people have been jailed as material witnesses since the attacks on the World Trade Center and the Pentagon.

In a rebuke of Attorney General John Ashcroft, U.S. District Judge Shira Scheindlin said: "Relying on the material-witness statute to detain people who are presumed innocent under our Constitution in order to prevent potential crimes is an illegitimate use of the statute."

Scheindlin threw out perjury charges against Osama Awadallah, 21, a Grossmont College student in El Cajon, Calif., who was accused of lying about his associations with two of the Sept. 11 hijackers.

At a news conference in Washington, Ashcroft defended the roundup of material witnesses after Sept. 11, saying the opinion of "one trial judge in New York represents an anomaly."

"The department's use of material witness warrants is fully consistent with the law and long-standing practice. Numerous other judges have authorized the use of material witness warrants in the settings that we have been using them, and the use of such warrants has been validated at the appellate level," he said.

In New York, U.S. Attorney James B. Comey said: "We believe the court's opinions are wrong on the fact and the law and we are reviewing our appellate options."

Scheindlin said the Founding Fathers who wrote the Bill of Rights believed people "should forever 'be secure in their persons, houses, papers and effects' from intrusion and seizure by officers acting under the unbridled authority of a general warrant."

Congress in 1984 carefully carved out an exception with a statute that lets a material witness be detained until his testimony can be secured by deposition in the pretrial phase of a court case, she said. The exception is clearly limited only to material witnesses in the pretrial phase of a criminal proceeding, rather than during a grand jury probe, she said.

"Detaining Awadallah solely for the purposes of a grand jury investigation was therefore unlawful," Scheindlin wrote. "Such an interpretation poses the threat of making detention the norm and liberty the exception."

Scheindlin cited the case of Abdallah Higazy, an Egyptian-born student arrested Dec. 17 as a material witness after a pilot radio was found in his Sept. 11 hotel room overlooking the World Trade Center. Charges were dropped when it was later found that the radio belonged to someone else and a hotel security guard had lied.

Scheindlin said that in Awadallah's case, a magistrate in San Diego not only ignored pertinent portions of the statute but added language to keep Awadallah imprisoned.

FBI agents confronted Awadallah outside his San Diego home on Sept. 20 and detained him a day later as a high-security prisoner.

"Having committed no crime -- indeed, without any claim that there was probable cause to believe he had violated any law -- Awadallah bore the full weight of a prison system designed to punish convicted criminals as well as incapacitate individuals arrested or indicted for criminal conduct," she said.

The judge also threw out evidence in the Awadallah case, including videotapes and a picture of Osama bin Laden.

"Wonderful," said Jesse Berman, a lawyer for Awadallah. "He did everything he could to be cooperative and they treated him terribly. I'm just happy that he's been vindicated."

Scheindlin's ruling followed a February hearing in which federal agents and Awadallah testified about the circumstances of his arrest.

Awadallah was charged with perjury, accused of lying about his knowledge of one of the hijackers in the suicide attack on the Pentagon.

In grand jury appearances, Awadallah admitted meeting hijacker Nawaf al-Hazmi 30 to 40 times but denied knowing associate Khalid al-Mihdhar. Confronted with an exam booklet in which he had written the name Khalid, he later admitted he knew both of them.

If convicted, Awadallah could have gotten up to 10 years in prison.