The leader of the 1990s ethics investigation into misdeeds by then-House Speaker Newt Gingrich rejected the idea Monday of releasing additional information from the probe, saying the 15-year-old published report covers all the pertinent aspects of the case.

Gingrich’s main rival for the GOP presidential nomination, Mitt Romney, on Monday called for the release of more material, suggesting that it may show evidence of further infractions by Gingrich.

But former representative Nancy Johnson (R-Conn.), who oversaw the two-year investigation, which led to a $300,000 fine and a reprimand of Gingrich, said the 1,200-page report issued by the House Ethics Committee is a complete and comprehensive overview of the case.

“The final report provided all the evidence in the case. The committee never releases additional material. I believe the report, as did the hearing, covered every aspect of the matter,” Johnson, now a health-care lobbyist for Baker Donelson, told The Washington Post in an e-mail.

In an effort to raise questions about Gingrich’s ethical fitness for the Oval Office, the Romney campaign has called for the release of all investigative files from the probe, suggesting that Republican voters need to fully vet the issue, along with Gingrich’s post­congressional work for health-care companies. On Monday, Romney himself joined in that line of attack.

“We could see an October surprise a day from Newt Gingrich,” he told reporters in Florida. “And so let’s see the records from the ethics investigation, let’s see what they show. Let’s see who his clients were at the time he was lobbying Republican congressmen for Medicare Part D.”

Romney’s request, however, would require the current Ethics Committee to vote to release any remaining supporting documents that were not part of the public report that came out in January 1997. According to Johnson, those documents would not include any additional material relevant to what is already online at the committee’s Web site. She added that the eight-member panel had an extensive amount of time to question Gingrich about any matter.

“The committee members had the freedom to question the witness as long as they had any unanswered questions,” Johnson said Monday.

The investigation was a landmark moment in the congressional ethics wars that began in the late 1980s — battles begun in large part by Gingrich when he was in the Republican minority. The Gingrich probe was prompted by a complaint from his 1994 Democratic opponent, who alleged that the Georgia Republican was violating tax laws through his use of nonprofits and a college course he was teaching.

The committee began its work as Gingrich ascended to the speaker’s chair after the 1994 midterm elections, and, seeking to avoid any appearance of conflict, it hired an outside counsel, James M. Cole, to conduct the investigation. Cole is now the deputy attorney general; Justice Department aides said Monday that he would not comment about the investigation.

The inquiry found that Gingrich violated tax laws and lied to the committee’s investigators, but lawmakers on the panel would not agree to those terms, instead deciding to declare the speaker “reckless” in the co-mingling of finances from the groups. “Mr. Gingrich ran a lot of very yellow lights,” Cole said in 1997 when he unveiled the report. “Orange lights. There were bells and whistles going off.”

The case centered around a course called “Renewing American Civilization” that Gingrich taught at Kennesaw State College in his congressional district outside Atlanta. He used the course to refine his plan to reclaim the GOP majority, and the committee found him guilty of not properly seeking legal advice about using money from foundations forbidden from participating in political causes.

Contents of the course were distributed nationally by the political committee that Gingrich was associated with, GOPAC. However, among what Gingrich has said were more than 1 million pages of documents turned over to the panel, the speaker’s lawyer in December 1994 told the committee that GOPAC had no involvement in the course.

After a lengthy negotiation, Gingrich agreed to the report’s findings that he “should have known” that those documents were not the full truth. The original draft included statements that he “knew or should have known” — which essentially accused him of lying to the committee.

The panel voted 7 to 1 to reprimand Gingrich, and on Jan. 21, 1997, the House voted 395 to 28to approve the deal, the first time a sitting speaker was publicly penalized by the Ethics Committee.

The $300,000 penalty was meant to cover the cost of the investigation, which was prolonged by the lengthy examination of whether the speaker had lied to the panel.

Among the eight committee members was future speaker Rep. Nancy Pelosi (D-Calif.).

Last month Pelosi sparked a minor fury — which the Romney camp is now seizing upon — when she suggested that there were “a thousand pages of his stuff” that had seemingly not been released. “I know a lot about him. I served on the investigative committee that investigated him, four of us locked in a room in an undisclosed location for a year,”she told Talking Points Memo, a liberal blog.

However, Pelosi’s aides later clarified that her “thousand pages” comment was referring to the report that is already in the public domain and that she had no secret files.

Soon after the Gingrich vote in 1997, the House’s Republicans and Democrats reached an unofficial detente in the ethics wars, leading to an eight-year run in which no lawmaker filed a complaint against another.