(WASHINGTON) -- An unidentified man entered national security adviser Jake Sullivan's Washington, D.C., home in late April, prompting a Secret Service investigation into how it could happen with agents supposedly on guard, according to an agency spokesperson.
Sullivan was home at the time and confronted the intruder, and the incident was over in a matter of minutes, according to sources familiar with what happened.
He had his first public reaction Wednesday, expressing confidence in the agency.
"I don't have any comment on the incident, but I have total faith in the Secret Service, and they do a remarkable job every day as professionals protecting people," Sullivan told reporters on Air Force One Wednesday afternoon as he accompanied President Joe Biden on his trip to Japan.
This incident occurred despite a detail of special agents outside the home who apparently never saw the man enter or leave the home. The man apparently entered through an unlocked door, according to sources.
The Secret Service immediately deployed technology that alerts when an unlocked door is opened, sources said, and a spokesman said the agency is examining the actions of the agents.
"Secret Service is examining a security incident that took place at a protectee site," Anthony Guglielmi, the Secret Service director of communications, said in a statement to ABC News.
"While the protectee was unharmed, we are taking this matter seriously and have opened a comprehensive mission assurance investigation to review all facets of what occurred," Guglielmi said. "Any deviation from our protective protocols is unacceptable and if discovered, personnel will be held accountable. Modifications to the protective posture have also been made to ensure additional security layers are in place as we conduct this comprehensive review."
A source familiar with the incident told ABC News that the man appeared to be intoxicated and didn't know exactly where he was, and it appeared that he didn't know he was in the home of the national security adviser. He was never apprehended.
Secret Service Director Kim Cheadle is "livid" over the breach, according to a source familiar with the investigation.
Sullivan, like most high-profile executive branch members, has an around-the-clock Secret Service detail that monitors outside of his Washington, D.C. residence.
The incident was first reported by the Washington Post.
The White House referred requests for comment to the Secret Service.
ABC News' Justin Gomez contributed to this report.