SEATTLE — An independent investigation has found that the corruption prosecution of former U.S. Sen. Ted Stevens, R-Alaska, was “permeated by the systematic concealment” of significant evidence that would have helped Stevens’ defense, the federal judge in the case revealed Monday.

Attorney and former prosecutor Henry F. Schuelke III, who conducted the probe, did not find technical grounds to file criminal contempt charges against the Justice Department attorneys who handled the case. But his report was silent on whether the government lawyers should be prosecuted for obstruction of justice.

That could be determined by a separate internal investigation by the Justice Department that is also, more than three years after Stevens’ trial, nearing its conclusion.

Judge Emmet G. Sullivan, who ordered the special prosecutor’s investigation when the case imploded in 2009, released a brief description of Schuelke’s 500-page report Monday but said the full document would not be made public until the Justice Department had reviewed it.

He made it clear that his concerns about the federal government’s handling of the case are substantial. Sullivan said “prosecutorial misconduct ... permeated the proceedings before this court to a degree and extent that this court had not seen in 25 years on the bench.”

The judge said Schuelke and his co-investigator, William B. Shields, “found evidence of concealment and serious misconduct that was previously unknown and almost certainly would never have been revealed — at least to the court and to the public — but for their exhaustive investigation.”

Stevens, who was the longest-serving Republican in the Senate, was found guilty in October 2008 — on the eve of his bid for re-election — of making false statements in connection with his relationship with Bill Allen, then-president of Veco Corp., an influential oil services company.

Stevens, who lost the election, died in a plane crash last year.

Only very late in the game did Stevens’ defense lawyers learn that a broad array of information casting doubt on Allen’s reliability as a witness and other important aspects of the trial was never turned over to the defense, as is required by law.

The Justice Department took the extraordinary step of dismissing the indictment, but defense lawyers for other Alaskan politicians targeted in the corruption probe soon found similar discovery violations, throwing the entire federal investigation, known as Operation Polar Pen, into disarray.

Schuelke’s investigation and the still-uncompleted internal Justice Department probe have dragged on far longer than anyone expected, a delay that has left unanswered crucial questions about whether federal attorneys knowingly withheld exculpatory evidence and, if so, whether senior Justice Department officials ordered junior attorneys to keep quiet about their concerns.

Nicholas Marsh, the young prosecutor from the department’s public integrity section who put together some of the cases that targeted Alaskan politicians, committed suicide in September 2010. He had been transferred out of his job and, friends have told the Los Angeles Times, had been waiting endlessly for a verdict on his conduct amid fears he would be targeted by superiors as a scapegoat.

Sullivan said Schuelke’s inquiry involved the review of 150,000 pages of documents and interviews with numerous witnesses, including 12 depositions. Although it officially covered only the cases of Stevens, the probe also of necessity involved gaining a “comprehensive understanding” of at least two other cases brought under the wide-ranging Alaska corruption probe, that of former Alaska House Speaker Peter Kott and ex-legislator Victor Kohring, Sullivan said.

Those cases initially were thrown out on appeal as a result of the same kind of problems that plagued the Stevens case. They concluded this fall, with both men pleading guilty to felony charges.

Kott’s lawyer, Michael Filipovic, said he had not seen Schuelke’s report and could not comment on it. “However, with respect to the issue of releasing the report into the public domain, I think it is important that there be a public record of the details of the misconduct that took place in these cases,” Filipovic told the Times.

Stevens’ attorney, Brendan V. Sullivan Jr., also said he could not comment on the report.

Judge Sullivan said Schuelke found no grounds to file criminal contempt charges against any of the prosecutors because the judge had never specifically ordered them to comply with their discovery obligations — in part because the judge had assumed, based on their assurances, that the prosecution team was complying with the law.

“I’m not going to write an order that says ‘follow the law.’ We all know what the law is,” the judge told prosecutors during the trial.

“It should go without saying that neither Judge Sullivan, nor any district judge, should have to order the government to comply with its constitutional obligations, let alone that he should feel compelled to craft such an order with a view toward a criminal contempt prosecution, anticipating its willful violation,” Schuelke wrote, according to Sullivan’s summary.

The internal probe conducted by the Justice Department’s Office of Professional Responsibility could recommend filing charges for obstruction of justice, or some lesser remedy, such as firing or referral to local bar associations. Or it could find there was no actionable wrongdoing at all.

U.S. Attorney General Eric Holder told a congressional committee this month that the internal inquiry was in its “last stages.” “There is a multi-hundred-page report that ... is just about finalized,” Holder said. He said the fact that he was “bothered by what happened” already led him to dismiss the case against Stevens.

“My hope is that we will be able to share as much of that report as we possibly can,” he said. Justice Department officials said they would have no comment on the report released Monday.