Washington ? Despite the Sixth Amendment’s guarantee of public trials, nearly all records are being kept secret for more than 5,000 defendants who completed their journey through the federal courts over the past three years.

Instances of such secrecy more than doubled from 2003 to 2005.

An Associated Press investigation found, and court observers agree, that most of these defendants are cooperating government witnesses, but the secrecy surrounding their records prevents the public from knowing details of their plea bargains with the government.

Most of these defendants are involved in drug gangs, though lately a very small number come from terrorism cases. Some of these cooperating witnesses are among the most unsavory characters in America’s courts – multiple murderers and drug dealers – but the public cannot learn whether their testimony against confederates won them drastically reduced prison sentences or even freedom.

In the nation’s capital, which has had a serious problem with drug gangs murdering government witnesses, the secrecy has reached another level – the use of secret dockets. For hundreds of such defendants over the past few years in this city, should someone acquire the actual case number for them and enter it in the U.S. District Court’s computerized record system, the computer will falsely reply, “no such case” – rather than acknowledging that it is a sealed case.

At the request of the AP, the Administrative Office of U.S. Courts conducted its first tally of secrecy in federal criminal cases. The nationwide data it provided the AP showed 5,116 defendants whose cases were completed in 2003, 2004 and 2005, but the bulk of their records remain secret.

“The constitutional presumption is for openness in the courts, but we have to ask whether we are really honoring that,” said Laurie Levenson, a former federal prosecutor and now law professor at Loyola Law School in Los Angeles. “What are the reasons for so many cases remaining under seal?

“What makes the American criminal justice system different from so many others in the world is our willingness to cast some sunshine on the process, but if you can’t see it, you can’t really criticize it,” Levenson said.

The courts’ administrative office and the Justice Department declined to comment on the numbers.

The data show a sharp increase in secret case files over time as the Bush administration’s well-documented reliance on secrecy in the executive branch has crept into the federal courts through the war on drugs, anti-terrorism efforts and other criminal matters.