Scott Blackmun has been the CEO of the U.S. Olympic Committee since 2010. (FRANCIS VACHON/AP)

Two U.S. senators called for U.S. Olympic Committee chief executive Scott Blackmun to resign Friday over the Larry Nassar sex abuse scandal.

Sens. Jeanne Shaheen (D-N.H.) and Joni Ernst (R-Iowa) released a joint statement calling for Blackmun to step down in response to a Wall Street Journal report Thursday that Blackmun was informed in July 2015 by then-USA Gymnastics chief executive Steve Penny of sexual assault allegations against Nassar, the former longtime physician for Team USA women's gymnasts. Penny contacted Blackmun, according to the Journal, shortly before he reported Nassar to the FBI.

This timeline has been established since December, when the USOC acknowledged in response to a lawsuit that it knew of allegations against Nassar in 2015, when USA Gymnastics reported Nassar to the FBI, but Shaheen and Ernst termed Thursday's report "deeply disturbing."

"If these reports are true, this goes far beyond negligence and raises serious questions of culpability at USOC, in which the most appropriate action would be for Scott Blackmun to resign," the senators wrote in a joint statement.

In a statement Friday, USOC spokesman Mark Jones wrote: "We have consistently said we learned in 2015 of a doctor potentially having abused an athlete and that it was reported to the FBI. That's what is supposed to happen."

After Penny reported Nassar to the FBI's Indianapolis office in July 2015, USA Gymnastics quietly separated from Nassar, who served as a volunteer while working full time at Michigan State. The FBI's investigation languished, for reasons the bureau has never publicly disclosed, and Nassar continued to treat — and assault — his patients under the guise of pain therapy until August 2016, when a woman filed a complaint with Michigan State police and told her story to the Indianapolis Star.

In early 2017 — as the number of girls and women accusing Nassar of abuse topped 100, including multiple former Team USA gymnasts — Penny drew intense criticism for waiting five weeks after first hearing allegations against Nassar in 2015 to report them to the FBI and for not informing Michigan State or law enforcement in Michigan of allegations against Nassar. Last March, under pressure from the board of directors of the USOC, Penny resigned.

Late last year, Nassar pleaded guilty to 10 counts of sexual assault in Michigan. Last month, a judge sentenced Nassar, 54, to serve 40 to 175 years in prison after a lengthy seven-day sentencing hearing featuring emotional victim's impact statements from more than 160 girls, women and parents re-ignited national outrage over the Nassar case.

Shaheen and Ernst are among several members of Congress who have called for independent inquiries into the culpability of USA Gymnastics, the USOC and Michigan State in Nassar's crimes. The USOC also has announced plans for an inquiry.

"As we committed to the victims and survivors last week, we are launching 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," Jones wrote. "We are also committed to ensuring a process that is transparent, sensitive and accessible to those who wish to provide input to the independent investigator."

Friday evening, USOC board chairman Larry Probst issued a statement of support for Blackmun, while announcing international law firm Ropes & Gray would "conduct an investigation of how an abuse of this proportion could have gone undetected for so long."

"The results will be made public, and the USOC's board will act in accordance with those results, whatever they may be. Scott has served the USOC with distinction. . . . We understand the Senators' concerns and respect their views, but given the ongoing investigation the board believes that it is inappropriate and unfair to judge Scott before board members understand all of the facts," Probst wrote.

Blackmun, 60, has led the USOC since 2010. He previously served as the organization's general counsel in 1999 and 2000 and, briefly, as acting chief executive in 2001. After being passed over for the full-time CEO spot that year, he became chief operating officer for Anschutz Entertainment Group, a sports and music entertainment company.

Since returning to the USOC in 2010, Blackmun has drawn praise for repairing the organization's relationship with the International Olympic Committee, which recently awarded the 2028 Summer Games to Los Angeles. He has drawn sustained criticism over the years, however, from victims and advocates for perceived inaction on sex abuse scandals that have roiled national governing bodies, including USA Swimming, USA Taekwondo, U.S. Speedskating and USA Judo.

Blackmun has defended his actions by pointing to measures enacted since he returned in 2010, including the 2014 requirement of basic child protection measures such as criminal background checks and abuse education programs throughout Olympic national governing bodies and the establishment last year of an independent nonprofit — the U.S. Center for SafeSport — which has taken over responding to suspicions of abuse in Olympic sports organizations.

Headquartered in Colorado Springs and established by a federal charter, the USOC oversees 47 Olympic and Pan-American national governing bodies spread across the country and is in charge of assembling and managing Team USA for international competitions, including the Summer and Winter Olympics, Paralympics and Pan-American Games. The USOC receives no federal funding but averages $230 million in annual income derived largely from marketing and sponsorships, as it holds exclusive rights to the Olympic mark and related symbols in the United States.

In 2016, according to the organization's most recent annual filing with the IRS, Blackmun earned $1 million in total compensation. He recently disclosed he has been diagnosed with prostate cancer and is under treatment and, as a result, is not expected to be in attendance for the Winter Games in PyeongChang, South Korea, which begin next week.