The U.S. Secret Service reported an incident at the north side of the White House Monday morning just as President Trump and first lady Melania Trump were expected to return to the property from the Pentagon.

"Secret Service agent with TV pool says the White House lockdown was caused by someone who threw a sign and yoga mat over the fence," NBC reporter Peter Alexander tweeted.

Minutes earlier, the Secret Service reported that pedestrians were being blocked from Pennsylvania Avenue while the incident was being investigated.

"Uniformed Division responding to an incident at the north fence line, along Penn. Ave. Area is closed to pedestrian traffic," the Secret Service tweeted around 10:30 a.m. Washington time.

Uniformed Division responding to an incident at the north fence line, along Penn. Ave. Area is closed to pedestrian traffic. — U.S. Secret Service (@SecretService) September 11, 2017

Two people were initially reported being taken into custody for "throwing unknown objects" over the fence, the agency later added. The D.C. Fire and Police Departments both responded to the scene.

The Secret Service issued an update that a man was arrested by D.C. police.

"Items thrown were a sign and a notebook. Both were deemed nonhazardous," the agency said.

Incident cleared, male subject arrested & taken to @DCPoliceDept. Items thrown were a sign and a notebook. Both were deemed nonhazardous. — U.S. Secret Service (@SecretService) September 11, 2017

It's not clear whether the Trumps had returned from a 9/11 observance ceremony at the Pentagon at the time of the disturbance.

The Trumps were scheduled to leave the Pentagon at 10:20 a.m. and arrive at the White House at 10:35 a.m.

Pennsylvania Avenue and Lafayette Park remain closed as of 11 a.m.