Silence of terror suspects frustrates FBI
US investigators say they are increasingly frustrated by the silence of detained terrorist suspects.
More than 150 people rounded up after the September 11 attacks remain in custody.
The FBI believes four held in New York have particularly useful information.
Agents say offers of lighter sentences, jobs, money and new identities have not prompted a response.
Reports claim an FBI source said: "We're into this thing for 35 days and nobody is talking. Frustration has begun to appear."
One agent said: "We are known for humanitarian treatment, so basically we are stuck. Usually there is some incentive, some angle to play, what you can do for them. But it could get to that spot where we could go to pressure, where we won't have a choice - and we are probably getting there."
Under US law, interrogators in criminal cases can lie to suspects, but information obtained by physical pressure, inhumane treatment or torture cannot be used in a trial.
In addition, the government interrogators who used such tactics could be sued by the victim or charged with battery by the government.
The four key suspects are being held in New York's Metropolitan Correctional Centre.
They are Zacarias Moussaoui - a French Moroccan detained in August after he sought flying lessons; Mohammed Jaweed Azmath and Ayub Ali Khan - Indians traveling with false passports, box cutters, and hair dye; and Nabil Almarabh - a former Boston cab driver with alleged links to al-Qaida.
