The head of the Secret Service asserted Thursday that the security breach at last week's White House state dinner was an aberration but has put three uniformed officers on leave.
President Barack Obama says his confidence in the agency remains unshaken.
The chairman of the House Homeland Security Committee, Rep. Bennie Thompson, said the country is fortunate the affair in honor of visiting Indian Prime Minister Manmohan Singh did not end in a "night of horror."
Meantime in an interview at the White House, Obama said that even though "the system didn't work the way it was supposed to" last week, he still feels safe in the mansion and trust the agency fully to protect not only him but his wife and children.
"I could not have more confidence in the Secret Service," Obama told USA TODAY and the Detroit Free Press.
Appearing before Thompson's panel for questioning, Sullivan acknowledged his officers made mistakes and that the Secret Service must have a "100 percent" performance record.
Thompson, a Democrat, also said that Congress needs to talk not only to Tareq and Michaele Salahi, the couple who got in without invitations, but also to White House social secretary Desiree Rogers. All three have declined to appear. Thompson said he is likely to authorize a subpoena for the Salahis to testify.
Rep. Pete King, the panel's top Republican, accused the White House of "stonewalling" in not permitting Rogers to appear. The conngressman said he thinks the White House is either afraid of something or doesn't want to take any heat for last week's incident.
Thompson said at midday that the Salahis could be cited for contempt of Congress if they continue to shun the committee's request that they proceed. He said he has asked staff to prepare subpoenas for the pair, and said "my door remains open." King indicated he would continue to press for Rogers to appear as well. The two appeared to differ on whether Rogers should be subpoenaed.
Thompson said at the outset: "This hearing is not about crashing a party at the White House. Nor is it about wannabe celebrities." He said the purpose is to better protect the president.
The Salahis have been trying to land a part on a telvision reality show, "The Real Housewives of D.C.," and were filmed by the TV show around town as they prepared for the White House dinner.
"We're not concerned about agency embarrassment," Thompson said. "We're all fortunate that this diplomatic celebration did not become a night of horror. ... We must dissect every fact ... and after we do these things, we need to give thanks that no lives were lost," he said.
Said Sullivan: "In our judgment, a mistake was made. In our line of work, we cannot afford even one mistake."
"I fully acknowledge that the proper procedures were not followed," he said. " ... This flaw has not changed our agency's standard, which is to be right 100 percent of the time."
Thompson asked Sullivan what went wrong.
"Pure and simple, this was human error" in which normal security protocols were not followed, Sullivan said. The breach was not caused by poor screening technology, he added.
The Secret Service chief said the investigation so far has found three people from the agency's uniformed officer division responsible for the security breach and all three have been put on administrative leave. He added that the agency is still reviewing what security protocols weren't followed.
"What we find is if the protocols are followed, we would not run into this situation," Sullivan said.
Asked whether there was a risk posed to people attending the dinner for the visiting prime minister of India, Sullivan said he was confident there wasn't.
Sullivan said there was no threat to Obama, noting that "last week we took him to a basketball game, and there was 5,000 people sitting around the president."
In response to a question he said Obama had not had an extraordinary number of threats against his life, contrary to her assertion, and said that Obama had received no more such threats at this point in his term than his two predecessors.
White House press secretary Robert Gibbs earlier this week described both Obama and his wife, Michelle, as angered by the incident.