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.

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.