The Secret Service says it is investigating an intrusion that occurred at the residence of National Security Adviser Jake Sullivan in late April. Sullivan, a close aide of President Biden, was unharmed during the incident.

According to sources who spoke to The Washington Post, the intrusion occurred around 3 a.m. a few weeks ago after an unknown man walked into Sullivan’s home in Washington, D.C.

Sullivan confronted the intruder, who appeared to be intoxicated, sources said. No signs of forced entry were discovered and Sullivan instructed the man to leave, they added.

The Secret Service said it considered the security breach a matter of significant concern.

“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,” Secret Service spokesman Anthony Guglielmi said in the statement, in response to an inquiry from The Washington Post.

“Any deviation from our protective protocols is unacceptable and if discovered, personnel will be held accountable,” the statement added.

The motives behind the recent intrusion at Sullivan’s home were not immediately clear, however the incident occurs amidst growing concerns over plots to attack U.S. officials and politicians.

Last October, Paul Pelosi was reportedly assaulted after a man entered his San Francisco residence while looking for his wife, former House Speaker Nancy Pelosi. The home was reportedly under Capitol Police surveillance.

Months before that, authorities arrested an armed man near the home of Supreme Court Justice Brett Kavanaugh. The man had reportedly set out to kill the justice.

Although the justices were already provided with round-the-clock protection by the U.S. Marshals Service, Congress subsequently extended security measures to include the families of the justices.

The White House declined to comment on the specific incident at Sullivan’s residence.

The Secret Service says it has deployed additional security precautions for Sullivan and around his home, pending the completion of the investigation.