Richard Smith, former chairman and chief executive officer of Equifax Inc., waits to begin a House Energy and Commerce Committee hearing in Washington, D.C., U.S., on Tuesday, Oct. 3, 2017.

It took weeks for Equifax insiders to understand the breadth and depth of the giant data breach they disclosed last month, former CEO Richard Smith told Congress on Tuesday, describing the around the clock investigation the company began in late July, when it first detected suspicious activity.

It wasn't immediately clear there was a breach, Smith told Congress. But eventually the company would disclose that criminals had accessed its systems and compromised the personal information of more than 145 million people.

A combination of human and technological error led to the hack, Smith said, adding he takes "full responsibility."

Smith made the remarks at a hearing of the House Energy and Commerce Committee. It is his first of four Capitol Hill stops this week as lawmakers investigate what happened and how it might have been prevented.

"The criminal hack happened on my watch," Smith said. "I am truly and deeply sorry for what happened."