U.S. Secret Service Director Julia Pierson called an incident in which a man scaled a fence and entered the White House "unacceptable" and told a congressional committee Tuesday that she takes "full responsibility."

Omar Gonzalez, a 42-year-old knife-wielding Iraq war veteran, entered an unlocked door to the executive mansion, Pierson told the House Committee on Oversight and Government Reform, which grilled her on the September 19 security breach.

A federal grand jury on Tuesday indicted Gonzalez on three separate federal and District of Columbia charges that could carry a maximum of 17 years in prison. His first court appearance is expected Wednesday in Washington.

The White House door through which Gonazlez barged had no remote locking mechanism and needed to be manually locked, Pierson told lawmakers, adding that such a mechanism has since been installed.

"It's clear that our security plan was not properly executed," she said. "I take full responsibility. What happened is unacceptable, and it will never happen again."

Saying she couldn't give complete responses because presidential protection is highly sensitive or classified, Pierson said the incident remains under investigation, and she doesn't "want to get ahead of the investigation."

It didn't come up at the hearing, but the Washington Post reported that the officer who tackled Gonzalez wasn't even on duty but was leaving a shift protecting the president's daughters.

Republicans and Democrats questioned how Gonzalez penetrated "five rings of security" in jumping the White House fence, overpowering a Secret Service officer and running deep into the White House, where he was finally subdued.

"How on earth did this happen?" asked Chairman Darrell Issa. "Why was there no guard stationed at the front door of the White House, and yes, how much would it cost to lock the front door of the White House?"

Added Rep. Elijah Cummings, the top Democrat on the panel, "I hate to even imagine what could have happened if Gonzalez had been carrying a gun instead of a knife when he burst inside the White House. That possibility is extremely unsettling."

Rep. Jason Chaffetz asked whether officers who caught Gonzalez acted appropriately by not using lethal force. Pierson said the Secret Service agents "did use restraint in making a very difficult decision" but conceded that she didn't believe the security plan was properly executed, which is why the incident is being reviewed.

The Secret Service should take a more aggressive posture, not a restrained one, Chaffetz said, and if agents kill someone who has breached White House security, "I will have their back."

Former Secret Service Director Ralph Basham later said that neither he nor the American public wants the Secret Service's first reaction at seeing someone invade the White House lawn to be: kill that person.

But after 3½ hours, it was clear that the committee wasn't confident in the agency's own review. Issa concluded the hearing by saying he and Cummings agreed that "an internal investigation by the secret service is not sufficient" and said they plan to formally request an independent inquiry.

(CNN)