National security adviser Jake Sullivan speaks during the daily briefing at the White House on April 24, 2023, in Washington, DC.
CNN  — 

The US Secret Service is investigating how an intruder entered US national security adviser Jake Sullivan’s home last month without being detected by Secret Service agents guarding his home.

Sullivan, who has 24/7 Secret Service protection, was unharmed in the incident, according to Secret Service spokesman Anthony Guglielmi, who said the agency takes the matter seriously.

Sullivan encountered the person inside his Washington, DC, home in the early morning hours one day in late April, according to a source familiar with the matter, and he told investigators he believed the person was intoxicated.

There was no threat made, the source said, and the person left Sullivan’s home without the Secret Service detail noticing.

The Washington Post first reported the incident.

Secret Service director Kimberly Cheatle is “not happy” with the undetected intrusion and ordered the investigation into the incident, a Secret Service official told CNN Wednesday.

“The Director is not happy with what transpired and ordered the mission assurance probe to gather all available facts before taking any decisive actions,” the official said, adding that personnel involved in the incident had been placed on administrative assignments while the review is ongoing.

Secret Service spokesman Anthony Guglielmi on Tuesday declined to offer further details about the incident itself until the agency’s internal investigation has concluded.

“An intrusion at the home of a protectee is extremely serious and it demands a meticulous inspection of all personnel, technology and processes to determine how such an aberration could occur,” Guglielmi said in a statement.

“We are currently in the process of conducting that comprehensive mission assurance investigation, predicated on facts and evidence. We respect the impartiality of this review and will not make any definitive statements until we have a complete and validated account of what occurred and how.”

This story has been updated with additional reporting.