WASHINGTON » Secret Service Director Julia Pierson abruptly resigned Wednesday in the face of multiple revelations of security breaches, bumbling in her agency and rapidly eroding confidence that the president and his family were being kept safe.

President Barack Obama "concluded new leadership of that agency was required," said spokesman Josh Earnest.

High-ranking lawmakers from both parties had urged her to step down after her poorly received testimony to Congress a day earlier — and revelation of yet another security problem: Obama had shared an elevator in Atlanta last month with an armed guard who was not authorized to be around him.

That appeared to be the last straw that crumbled trust in her leadership in the White House. Earnest said Obama and his staff did not learn about that breach until just before it was made public in news reports Tuesday.

"Today Julia Pierson, the director of the United States Secret Service, offered her resignation, and I accepted it," Homeland Security Secretary Jeh Johnson said in a statement. He announced that Joseph Clancy, retired head of the agency’s Presidential Protective Division, would come out of retirement to lead the Secret Service temporarily.

Taking further steps to restore trust in the beleaguered agency, Johnson also outlined an independent inquiry into the agency’s operations.

That trust was shaken by a series of failures in the agency’s critical job of protecting the president, including a breach Sept. 19, when a knife-carrying man climbed over the White House fence on Pennsylvania Avenue and made it deep into the executive mansion before being stopped.

Republicans quickly served notice that Pierson’s resignation and the inquiry ordered by Johnson would not end their investigation.

"The Oversight Committee will continue to examine clear and serious agency failures at the Secret Service," said the panel’s chairman, Rep. Darrell Issa, R-Calif. "Problems at the Secret Service pre-date Ms. Pierson’s tenure as director, and her resignation certainly does not resolve them."

In an interview with Bloomberg News after her resignation was announced, Pierson said she recognized that "Congress has lost confidence in my ability to run the agency."

She said she met Johnson on Wednesday and "after that discussion I felt this was the noble thing to do."

Rep. Jason Chaffetz, R-Utah, a leader of the congressional inquiry, called her resignation "the right thing to do, it had to happen, but there are some systemic challenges that must be addressed."

Some revelations came from whistleblowers who contacted Chaffetz, and he suggested more damaging stories may emerge. "Unfortunately there are more out there and we’ll see how that goes," he said.

After a congressional hearing Tuesday into the Sept. 19 breach and an earlier one, reports emerged of still another. Earlier in September, Obama had shared an elevator in Atlanta with a private guard who was not authorized to be around him with a gun. That was the first known Secret Service failure to unfold in the presence of the president. The first family was not at the White House when the recent intruder entered.

The White House learned about the Atlanta episode only about when lawmakers and the public did — when the Washington Examiner and The Washington Post reported it, Earnest said.

Obama had not been told about it previously, Earnest said. This, despite Pierson’s statement to the committee that she briefs the president "100 percent of the time" about threats to his personal security and those at the White House. She said the only time she had briefed him this year was after the Sept. 19 White House intrusion.

The man accused of running into the White House on Sept. 19, Omar J. Gonzalez, pleaded not guilty Wednesday in a brief appearance in federal court. He is accused of unlawfully entering a restricted building while carrying a deadly weapon, which is a federal charge, and two violations of District of Columbia law.

Wearing a standard prison-issue orange jump suit, Gonzalez sat attentively at the defense table but did not address the court as his lawyer entered the plea.

As for Pierson, support for the Secret Service director unraveled quickly after her defensive testimony Tuesday, which left key questions unanswered.

Rep. Elijah Cummings, top Democrat on the House Oversight Committee, said in multiple interviews Wednesday that Pierson was no longer the best person to lead the Secret Service.

She is the latest administration official to leave in the midst of controversy. Others include:

— Veterans Affairs Secretary Eric Shinseki, who resigned in May, taking the blame for what he decried as a "lack of integrity" in the sprawling health care system for the nation’s military veterans.

— Health and Human Services Secretary Kathleen Sebelius, who, as the "Obamacare" insurance marketplace failed spectacularly in its launch, stayed on to oversee repairs before Obama accepted her resignation months later.

Associated Press writers Erica Werner, Josh Lederman and Calvin Woodward contributed to this report.