FBI Director Christopher Wray was asked to explain what went wrong during the bureau's investigation of the Larry Nassar abuse claims. He responded, "I don't have a good explanation."
"It is utterly jarring to me. It is totally inconsistent with what we train our people on. Totally inconsistent from what I see from the hundreds of agents who work these cases every day," he added.
Wray reiterated that an agent involved in the investigation has been fired.
He said that the FBI agents who handled this case "betrayed core duty" of protecting people.
The Senate hearing is ongoing, with lawmakers on the judiciary committee questioning Wray and Justice Department Inspector General Michael Horowitz.
More context: Allegations into Nassar were first brought to the agency in July 2015. Several violations of protocols led to months of delay, as captured in a scathing Justice Department inspector general report released in July.
In recent weeks, an FBI agent accused in the inspector general's report of failing to launch a proper investigation was fired by the FBI, US law enforcement officials told CNN. A supervisor who was also singled out in the IG report for violating protocol and false statements retired from the FBI in January 2018.
CNN's Tierney Sneed contributed reporting to this post.