More than 150 people who were picked up after September 11 remain in custody, with four men the focus of particularly intense scrutiny. But investigators have found the usual methods have failed to persuade any of them to talk.

Options being weighed include “truth” drugs, pressure tactics and extraditing the suspects to countries whose security services are more used to employing a heavy-handed approach during interrogations.

“We’re into this thing for 35 days and nobody is talking. Frustration has begun to appear,” a senior FBI official told The Washington Post.

Under US law, evidence extracted using physical pressure or torture is inadmissible in court and interrogators could also face criminal charges for employing such methods. However, investigators suggested that the time might soon come when a truth serum, such as sodium pentothal, would be deemed an acceptable tool for interrogators.

The public pressure for results in the war on terrorism might also persuade the FBI to encourage the countries of suspects to seek their extradition, in the knowledge that they could be given a much rougher reception in jails back home.

One of the four key suspects is Zacarias Moussaoui, a French Moroccan, suspected of being a twentieth hijacker who failed to make it on board the plane that crashed in Pennsylvania. Moussaoui was detained after he acted suspiciously at a Minnesota flying school, requesting lessons in how to steer a plane but not how to take off or land. Both Morocco and France are regarded as having harsher interrogation methods than the United States.

The investigators have been disappointed that the usual incentives to break suspects, such as promises of shorter sentences, money, jobs and new lives in the witness protection programme, have failed to break the silence.

“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 don’t have a choice, and we are probably getting there,” an FBI agent involved in the investigation told the paper.

The other key suspects being held in New York are Mohammed Jaweed Azmath and Ayub Ali Khan, Indians who were caught the day after the attacks travelling with false passports, craft knives such as those used in the hijackings and hair dye. Nabil Almarabh, a Boston taxi driver alleged to have links to al-Qaeda, is also being held. Some legal experts believe that the US Supreme Court, which has a conservative tilt, might be prepared to support curtailing the civil liberties of prisoners in terrorism cases.

However, a warning that torture should be avoided came from Robert Blitzer, a former head of the FBI’s counter-terrorism section. He said that the practice “goes against every grain in my body. Chances are you are going to get the wrong person and risk damage or killing them.”

In all, about 800 people have been rounded up since the attacks, most of whom are expected to be found to be innocent. Investigators believe there could be hundreds of people linked to al-Qaeda living in the US, and the Bush Administration has issued a warning that more attacks are probably being planned.

Newsweek magazine reports today that Mohammed Atta, the suspected ringleader who died in the first plane to hit the World Trade Centre, had been looking into hitting an aircraft carrier. Investigators retracing his movements found that he visited the huge US Navy base at Norfolk, Virginia, in February and April this year.