Secret Service investigating how an intruder entered national security adviser Jake Sullivan's home undetected

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.

Guglielmi said in a statement that the "Secret Service is examining a security incident that took place at a protectee site. 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."

"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," the spokesman said.

This story has been updated with additional reporting.