Since 9/11, the Justice Department has used a little-known legal tactic to secretly lock up at least 70 terror suspects--almost all of them Muslim men--and hold them without charges as "material witnesses" to crimes, in some cases for months. A report to be released this week by two civil-liberties groups finds nearly 90 percent of these suspects were never linked to any terrorism acts, resulting in prosecutors and FBI agents issuing at least 13 apologies for wrongful arrest.

The post-9/11 decision to aggressively use "material witness" warrants to detain suspects has been defended by Justice officials as a legitimate tool to root out possible terror cells. (A federal law, though used sparingly in the past, permits detention of witnesses who might have "material" info about a crime--even with no evidence they committed any crimes themselves.) The practice has been shrouded. Citing national security, Justice has refused to disclose virtually any info about these cases, not even figures on how many have been detained. By combing court records and interviewing defense lawyers, researchers for Human Rights Watch and the American Civil Liberties Union say they have assembled the most comprehensive look yet at the practice--and conclude it may have produced the most civil-liberties abuses of any post-9/11 policy. Out of the 70 "material witness" arrests the groups were able to document, only seven suspects ended up charged with terror-related crimes. Of the rest, 42 were released with no charges at all and another 20 were charged with unrelated crimes, such as credit-card fraud. (Two, Jose Padilla and Ali al-Marri, were named "enemy combatants" and thrown into military brigs.) The report cites instances in which agents used what it calls "flimsy" evidence to make arrests. A 68-year-old Virginia doctor named Tajammul Bhatti was arrested by the FBI in June 2002 after neighbors found magazines about flying and a phone number of a Pakistani nuclear scientist in his apartment. It turned out he had served in the U.S. Air Force National Guard and the Pakistani scientist was a childhood friend. Another "tip" led to the arrest of eight restaurant workers in Evansville, Ind., who were shackled and taken to a detention facility in Chicago. The FBI later apologized--but never disclosed the basis for their detention. "The law was never designed to be used this way," says Anjana Malhotra, the prime author of the report. Justice spokesman Kevin Madden called "material witness" detentions a "critical" tool to thwart crimes and cited recent testimony from a top official, Chuck Rosenberg, noting that every material-witness arrest warrant must be based on "probable cause" and approved by a federal judge. "Justice cannot unilaterally arrest someone as a material witness," Rosenberg said.