Strzok conceded during a joint hearing of the House Oversight and Judiciary committees that metadata indicates his computer made the change, but said he can't remember doing it.
The June 2016 edit changed "grossly negligent," a term that carries legal liability under the Espionage Act, to "extremely careless."
The new wording was uttered by then-FBI Director James Comey in July 2016 when he recommended against charging Clinton with a crime for using a private server to handle classified information while secretary of state.
Strzok, removed from special counsel Robert Mueller's Russia investigation last year after discovery of pro-Clinton and anti-Donald Trump text messages with then-FBI attorney Lisa Page, said someone else recommended the change.
"My recollection, sir, is that somebody within our office of general counsel did, it was one of the attorneys, I don't remember which one," Strzok said. "It was a legal issue that one of the attorneys brought up."
After a meeting, the change was made on Strzok's computer.
"I don't recall specifically when it happened," he testified.
Rep. Jim Sensenbrenner, R-Wis., noted that metadata indicated that Strzok's computer made the change.
"I am aware as well of that metadata," Strzok said. "My recollection is of working on the draft with a group of us in my office because it was the largest office and taking the inputs of probably five or ten different people."
Sensenbrenner asked Strzok to confirm that his computer made the change.
"Based on my subsequent review of that metadata, I believe that to be true," Strzok said.
Strzok said that only he had full access to the computer, but that "my secretary had access to elements of it."
Explaining the decision to soften the wording, he said, "my recollection was that attorneys within the FBI had raised the concern that the use of gross negligence triggered a very specific legal meaning ... particularly [in] one of the mishandling [classified information] statutes."
Sensenbrenner asked, "This change was Hillary's get out of jail free card, right?"
"She received no get out of jail free card, we pursued the facts," Strzok insisted.
"That rates four Pinocchios," Sensenbrenner told him.
Strzok refuted Sensenbrenner's conclusion. He said Comey "ultimately...made the decision to change that wording" following a "legal discussion of the use of that term."