Blackmun told investigators that he had called a meeting with experts and other U.S.O.C. officials after learning of the accusations, giving investigators names of several of the attendees, to initiate an internal review of the Nassar case. He said he wanted to make sure the organization was “doing the right thing.”

But the investigators from the law firm could not corroborate that story, and Blackmun later said he “was mistaken in his recollection about having taken such an effort,” according to the report.

The U.S.O.C. did nothing to investigate if Nassar had treated any athletes at the Olympics or any other U.S.O.C. event, nor did it look into whether the gymnastics federation was ensuring the safety of its athletes, according to the report. The U.S.O.C. also never followed up to see whether Penny, who resigned in March 2017 and was arrested two months ago on charges of evidence tampering, had reported Nassar to law enforcement.

The report also found that the U.S.O.C. did not report Nassar to law enforcement or take any steps to bar Nassar from U.S.O.C. events or facilities. It did not even discuss the allegations against Nassar with its own department that deals with sexual abuse cases.

At the same time, according to the report, Blackmun and Penny — who was leading one of the United States’ most successful Olympic sports — were working hard to “preserve their institutional interests — even as Nassar retired from the sport with his reputation intact and continued to have access to girls and young women at the college, club and high school levels.”

Blackmun did not return a phone call requesting comment late Monday.

While the report is very detailed, Senator Blumenthal said he was skeptical that it “actually presents the full picture” because there were so many questions left unanswered and documents that had gone missing.

Senator Jerry Moran, Republican of Kansas and chairman of the subcommittee, said in a statement that he was relieved that the report was now public, “if only so the U.S.O.C. can no longer cite an ongoing independent investigation as its reason for delaying change.”

Change, though, might be coming, no matter what the U.S.O.C. does. Senator Moran and Senator Blumenthal said that their subcommittee — which has been investigating sexual abuse in Olympic sports as well as what happened in the Nassar case — was on the cusp of recommending changes to the Ted Stevens Olympic and Amateur Sports Act, under which the U.S.O.C. operates.