The US indefinitely detained some 70 Muslim men after the 11 September attacks on baseless accusations of terrorist links, US rights bodies say.
The report says the US detained the men without good reason
The US Justice Department held the men under a federal law as witnesses likely to flee, Human Rights Watch and the American Civil Liberties Union say.
In a new report, the US is criticised for not arresting the men as criminal suspects, and later releasing many.
The Justice Department says the witness ruling is crucial to crime-fighting.
The material witness law was designed to allow the detention of witnesses thought to have information relating to a crime but who might flee.
Judges were willing to co-operate with FBI calls for detentions in the weeks and months after 11 September 2001 as authorities attempted both to investigate the attacks and to prevent fresh strikes.
In the report, the two human rights groups say the US government has "twisted" the material witness law "beyond recognition".
They accuse the US government of using the law to detain suspects when there was not enough substantive evidence to hold them for questioning.
Many were not informed of the reason for their arrest, allowed immediate access to a lawyer, or permitted to see the evidence against them, the report says.
"They didn't have access to the basis for their arrest, weren't provided with lawyers, weren't allowed to talk to family members and were held in complete secrecy with no concrete end to their detention," said the report's author, Anjana Malhotra.
A spokesman for the US Justice Department told the Associated Press that the material witness ruling was designed with judicial safeguards in mind and was a key part of fighting a range of crimes.
The report says arrests were often made at gunpoint and the men were held in solitary confinement and subjected to degrading treatment.
It says only 28 people were charged with offences, and just seven were charged with providing material support to terrorist bodies.
The US government has issued apologies to 13 of the detainees in question, the report adds.
Senator Patrick Leahy, the senior Democrat on the Senate Judiciary Committee, told AP he was concerned over possible misuse of the law and would consider narrowing its uses.