When suspected Al Qaeda operative Jose Padilla was whisked from the criminal justice system to military custody in June 2002, it was done for a key purpose – to break his will to remain silent.

As a US citizen, Mr. Padilla enjoyed a right against forced self-incrimination. But this constitutional guarantee vanished the instant President Bush declared him an enemy combatant.

For a month, agents of the Federal Bureau of Investigation had been questioning Padilla in New York City under the rules of the criminal justice system. They wanted to know about his alleged involvement in a plot to detonate a radiological "dirty bomb" in the US. Padilla had nothing to say. Now, military interrogators were about to turn up the heat.

Padilla was delivered to the US Naval Consolidated Brig in Charleston, S.C., where he was held not only in solitary confinement but as the sole detainee in a high-security wing of the prison. Fifteen other cells sat empty around him.

The purpose of the extraordinary privacy, according to experts familiar with the technique, was to eliminate the possibility of human contact. No voices in the hallway. No conversations with other prisoners. No tapping out messages on the walls. No ability to maintain a sense of human connection, a sense of place or time.

In essence, experts say, the US government was trying to break Padilla's silence by plunging him into a mental twilight zone. Padilla was not the only Al Qaeda suspect locked away in isolation. Although harsh interrogation methods such as water-boarding, forced hypothermia, sleep deprivation, and stress positions draw more media attention, use of isolation to "soften up" detainees for questioning is much more common.

"It is clear that the intent of this isolation was to break Padilla for the purpose of the interrogations that were to follow," says Stuart Grassian, a Boston psychiatrist and nationally recognized expert on the debilitating effects of solitary confinement. Dr. Grassian conducted a detailed examination of Padilla for his lawyers.

Padilla is on trial in Miami on charges that he became a willing recruit and attended an Al Qaeda training camp in Afghanistan. Padilla denies the allegations. Jury deliberations in the three-month trial are expected to begin this week.

Although the issue of Padilla's treatment in the brig arose briefly in the Miami case, no judge has ruled on its legality.

According to defense motions on file in the case, Padilla's cell measured nine feet by seven feet. The windows were covered over. There was a toilet and sink. The steel bunk was missing its mattress.

He had no pillow. No sheet. No clock. No calendar. No radio. No television. No telephone calls. No visitors. Even Padilla's lawyer was prevented from seeing him for nearly two years.

For significant periods of time the Muslim convert was denied any reading material, including the Koran. The mirror on the wall was confiscated. Meals were slid through a slot in the door. The light in his cell was always on.

Those who haven't experienced solitary confinement can imagine that life locked in a small space would be inconvenient and boring. But according to a broad range of experts who have studied the issue, isolation can be psychologically devastating. Extreme isolation, in concert with other coercive techniques, can literally drive a person insane, these experts say. And that makes it a potential instrument of torture, they add.

Interrogators say the whole point of an interrogation is to overcome a detainee's will to resist. Some try to build rapport. Others prefer a tougher approach.

"These are interrogations, they are not job interviews. So there has to be a certain amount of unpleasantness about it," says David DeBatto, a retired Army counter-intelligence agent and former interrogator. "You have to set the tone and the atmosphere. Some of that can include sensory deprivation, which means [the subject] is in a closed room, there is no sound, and he stays in there for various amounts of time." At that point, the interrogator must make a crucial judgment. "The question is: How long is too long? Is it a day? Is it a week? Is it a month?" Mr. DeBatto says.

When then-Defense Secretary Donald Rumsfeld approved isolation as an aggressive interrogation technique for use at Guantánamo Bay, Cuba, Defense Department lawyers included a warning. "This technique is not known to have been generally used for interrogation purposes for longer than 30 days," the April 2003 memo reads in part. Longer than that required Mr. Rumsfeld's approval.

By April 2003, Padilla had already spent 10 months in isolation at the brig. Ultimately, he was housed in the same cell, alone in his wing, for three years and seven months, according to court documents.

"I'm not a psychologist, but if he is not profoundly psychologically disturbed from that experience then he is a stronger man than me," says Steven Kleinman, a retired US Air Force Reserve colonel and former interrogator.

Padilla was visited by a military psychologist upon his arrival at the Charleston brig in early June 2002. The brief screening report says Padilla was not experiencing any mental-health concerns. But he didn't see another psychologist again for nearly two years, according to a report filed by psychologist Patricia Zapf of New York, who examined Padilla and his brig records at the request of Padilla's lawyers in his ongoing trial.

When the screening reports resumed in mid-2004, Padilla's mood is described as "anxious" and later as "elevated." There is no indication that he was given a full psychological evaluation, Ms. Zapf's report says. "In my opinion, it appears unusual that someone held in solitary confinement for upwards of three years would not have undergone a more thorough and regular evaluation of [his] mental state." The new Army Field Manual bars the use of isolation to achieve psychological disorientation through sensory deprivation. "Sensory deprivation is defined as an arranged situation causing significant psychological distress due to a prolonged absence, or significant reduction, of the usual external stimuli and perceptual opportunities," the manual states. "Sensory deprivation may result in extreme anxiety, hallucinations, bizarre thoughts, depression, and anti-social behavior. Detainees will not be subject to sensory deprivation."

Despite the tough words, the field manual offers only a general prohibition. So-called coercive interrogation methods – including isolation – have been specially authorized for certain units in the military and the Central Intelligence Agency.

The technique is not new. The Soviets used isolation and sensory deprivation to identify and discredit political dissidents. US prisoners of war confessed to nonexistent war crimes in the Korean War after similar treatment.

Fear of "brainwashing" prompted the CIA and Defense Department to underwrite research in the 1950s and '60s into the impact of isolation and sensory deprivation. The findings were included in a 1963 CIA handbook, later declassified. The book discusses the possible use of such techniques, including isolation. But it warns of the "profound moral objection" of applying "duress past the point of irreversible psychological damage."

That's what happened in Padilla's case, says Grassian. "It is clear from examining Mr. Padilla that that limit was surpassed."

He has "undergone a profound, tremendously prolonged psychological stress involving extended periods of utter isolation and deprivation," the psychiatrist writes. Grassian's report concludes: "Given the extensive research on this issue, much of it funded by the United States government, it follows necessarily that the United States government was well aware of the likely consequences of its conduct in regard to Mr. Padilla."

Officials have repeatedly said that the US doesn't use torture. "The Department of Defense policy is clear – we treat all detainees humanely," says Navy Cmdr. J.D. Gordon, a Defense Department spokesman. "The United States operates safe, humane, and professional detention operations for unlawful enemy combatants who are providing valuable information in preventing further terrorist attacks." He adds that no direct evidence has been presented that Padilla was tortured.

In preparing his report, Grassian studied the daily activity log maintained by staff members working at the brig. He discovered a particularly intense period of isolation from late November 2002 to early April 2003. Padilla has referred to this period as the "terrible time."

"It was not unusual for Mr. Padilla to go four, five, or six days without even brief [visual checks] by the brig staff, who were, in any event, under instruction not to converse with him," Grassian writes. Other than the brief checks by brig guards, Padilla went through stretches of 34 days, 17 days and 15 days without any human contact, the report says. "And when he did have such contact, it was inevitably with an interrogator," Grassian says.

That was by design. Vice Adm. Lowell Jacoby, director of the Defense Intelligence Agency, revealed a portion of the government's interrogation strategy in a 2003 court affidavit. "The Defense Intelligence Agency's approach to interrogation is largely dependent upon creating an atmosphere of dependency and trust between the subject and interrogator," he wrote. "Anything that threatens the perceived dependency and trust between the subject and interrogator directly threatens the value of interrogation as an intelligence-gathering tool." Admiral Jacoby wrote the affidavit to urge a New York federal court not to allow Padilla to consult his lawyer.

Grassian, the psychiatrist, disagrees with Jacoby's suggestion that Padilla's isolation would build trust: "What the government is attempting to do is create an atmosphere of dependency and terror."

The issue is much larger than the Padilla case. This weekend, the American Psychological Association will meet in San Francisco. A vocal part of the group is urging the APA to embrace a ban on detention and interrogation work by its members. Psychologists should use their knowledge and expertise for healing, they say.

Others counter that using one's professional expertise to help protect the country is an honorable motive. In addition, some say the presence of psychologists at detention and interrogation centers can help prevent abuses before they arise.