White House journalists and media crews were ordered by the Secret Service to clear the North Lawn of the building on Monday.

The security precaution happens on occasion in response to possible threats to the White House building, such as a suspicious package or a fence jumper.

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

Law enforcement officials are paying particularly close attention to security today as it is September 11, the anniversary of the 9/11 terrorist attacks.

President Donald Trump was at the Pentagon at the time of the security alert, delivering a speech recognizing 9/11.

NBC’s Peter Alexander noted on Twitter that a source told him that the White House lockdown was triggered after someone threw a yoga mat and a sign over the White House fence.

NEW: 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. — Peter Alexander (@PeterAlexander) September 11, 2017

The Secret Service said they arrested two individuals as a result of the incident.

2 individuals in our custody after throwing unknown objects over @WhiteHouse north fence line. @DCPoliceDept and fire responding. — U.S. Secret Service (@SecretService) September 11, 2017

Later the Secret Service said that the items were a notebook and a sign, not a yoga mat.