Of those 39 people, few had any connection to al-Qaida while the remaining 90% were acquitted or convicted of lesser crimes such as immigration violations or making false statements, the study shows.

The report emerged as President George Bush travelled the country to encourage Congress to renew the Patriot Act, the controversial attempt to counter terrorism by boosting surveillance powers.

"Federal terrorism investigations have resulted in charges against more than 400 suspects, and more than half of those charge have been convicted," Mr Bush told the Ohio State Highway Patrol Academy on Thursday.

But, according to the Washington Post, the vast majority of those arrested were eventually convicted of only minor violations for which they were either deported or received relatively short sentences.

The median sentence for all of the cases, related to terrorism or not, was 11 months. Of those who were convicted on terrorist charges, most were involved not with al-Qaida but causes and crimes such as Colombian drug cartels, Rwanda's civil war or support for Palestinian groups.

The administration's list does not include those held at Guantánamo Bay or under US jurisdiction elsewhere in the world.

"What we're seeing over time is the equivalent of mission creep," Juliette Kayyem, who heads the national security programme at Harvard University's Kennedy school of government, told the Post. "Cases that would not be terrorism cases before September 11 are swept on to the terrorism docket."

· Senator Mel Martinez said the government should consider closing the Guantánamo Bay prison for terrorism suspects - the first high-profile Republican to make the suggestion.