The U.S. Olympic Committee didn’t intervene in USA Gymnastics’ handling of sexual-abuse allegations against longtime national-team doctor Larry Nassar in 2015, even after USA Gymnastics’ then-president told two top USOC executives that an internal investigation had uncovered possible criminal behavior by the doctor against Olympic athletes.

The communications—a July 2015 phone call and a September 2015 email described to The Wall Street Journal by people familiar with the matter—shed new light on the Olympic Committee’s knowledge of a scandal that has since engulfed American gymnastics.

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The interactions also raise questions about why officials at USOC, which oversees USA Gymnastics and has criticized that organization’s response to the Nassar scandal, didn’t reach out to athletes, law enforcement or Dr. Nassar’s other employers in the year before allegations against him became public in September 2016.

During that yearlong period, a federal investigation into the matter languished, and Dr. Nassar continued to see—and allegedly abuse—patients in Michigan.

The USOC declined to comment on questions about its communications with USA Gymnastics. USA Gymnastics didn’t respond to requests for comment.

In a statement, USOC spokesman Mark Jones reiterated the committee’s plan, announced last week, to launch an “independent investigation into the decades-long abuse by Larry Nassar to determine what complaints were made, when, to whom, and what was done in response.”

The USOC has offered varying statements about when it first learned of the allegations against Dr. Nassar, and in what degree of detail. Last month, in response to a gymnast’s lawsuit, the organization said it was “first made aware of the possibility that a USA Gymnastics physician had sexually abused USA Gymnastics athletes in the summer of 2015, when we were informed by USA Gymnastics.” At that time, “USA Gymnastics indicated they were in the process of contacting the appropriate law enforcement agencies.”

In a June 2017 statement, USA Gymnastics’ leaders expressed their “deepest regrets” to athletes who were abused in the sport. The organization’s entire board of directors resigned in recent days, part of an overhaul of the group prompted by the Nassar scandal.

Around July 25, 2015, USA Gymnastics then-President Steve Penny called USOC Chief Executive Scott Blackmun with a request for guidance, according to a person familiar with the call. An Olympic gymnast had described, in a July 24 conversation with an internal investigator, what appeared to amount to sexual assault by a team doctor. At the investigator’s recommendation, Mr. Penny said he planned to report the matter to law enforcement, according to the person. ​

Mr. Blackmun told Mr. Penny to “do what he had to do,” the person familiar with the call said. Mr. Blackmun provided no further guidance to USA Gymnastics on the matter in the months to come.

Two months later, Mr. Penny emailed the USOC’s longtime chief security officer, Larry Buendorf, detailing three top gymnasts’ allegations against Dr. Nassar, including a graphic description of a purported treatment Dr. Nassar used, which at least one gymnast said involved inserting his finger into her vagina. The email was described to the Journal by people who have reviewed the correspondence.

The email appears to be the first documented instance in which a USA Gymnastics official mentions Dr. Nassar by name to a USOC official.

The USOC has previously said nobody at the organization knew Dr. Nassar was the alleged abuser until media reports in September 2016. Dr. Nassar was arrested in November 2016 on sexual-abuse charges in Michigan and was sentenced last week by a Michigan judge to up to 175 years in prison on some of those charges.

In an email Wednesday, Mr. Blackmun said he had encouraged Mr. Penny to turn the matter over to law enforcement, and he said the organization’s coming independent investigation would address questions of who knew about Dr. Nassar and when. Mr. Buendorf couldn’t be reached to comment.

Mr. Penny, who declined to comment, resigned from USA Gymnastics last year under pressure from USOC officials. At the time, Mr. Penny said he was leaving “to support the best interests of USA Gymnastics,” adding that the allegations of abuse were “heartbreaking.”

While USA Gymnastics and Michigan State University—where Dr. Nassar worked for two decades—have received the brunt of public criticism for the Nassar case, others have begun pointing fingers at the USOC. U.S. lawmakers have called for an investigation into both athletic organizations.

Gold-medal-winning gymnast McKayla Maroney, who was among the first victims identified by USA Gymnastics in the summer of 2015, last month sued the USOC, along with others, saying that the organization failed to protect athletes and that top USOC officials lied about when they first learned of the matter.

Last week, following Dr. Nassar’s sentencing, Mr. Blackmun apologized to the national team, conceding that the abuse detailed in the sentencing hearing “was worse than our own worst fears.”

In June 2015, a coach at the Karolyi Ranch training center in Texas overheard Olympic gymnast Aly Raisman and former national-team member Maggie Nichols discussing concerns about Dr. Nassar’s treatment, according to USA Gymnastics. The coach reported the matter to USA Gymnastics officials, who hired an outside investigator.

Five weeks later, Mr. Penny called Mr. Blackmun, and three days later, on July 27, Mr. Penny reported the matter to the FBI office in Indianapolis. Dr. Nassar was quietly pushed out of the organization that summer and announced his retirement from USA Gymnastics.

USA Gymnastics has said the FBI instructed the organization not to speak about the allegations to avoid compromising the investigation. The FBI has declined to comment on that assertion.

The September email from Mr. Penny to Mr. Buendorf outlined USA Gymnastics’ handling of the matter, including the work done in July 2015 by the investigator.

Mr. Penny told Mr. Buendorf in the email that three national-team gymnasts described discomfort with Dr. Nassar’s treatment—a technique that Dr. Nassar said had some basis in clinical practice but which was described by at least one of the gymnasts as “rougher,” “aggressive,” and perhaps solely for his sexual gratification, the people familiar with the email said.