A September 2013 incident in which a drone crashed in front of German Chancellor Angela Merkel during a campaign event reinforced the potential dangers posed by small drones, like the one that flew onto White House grounds early Monday.

Activists from Germany’s Pirate Party took credit for the one in Dresden, saying they were protesting Merkel’s surveillance policies.

The Secret Service has been concerned about drones entering White House airspace. Despite all the post-9/11 efforts to secure the area, drones are still a problem because radar often doesn’t spot their movement. Officials familiar with the air protection at the White House said the threat of a drone carrying explosives or surveillance is not easily stopped, but is a high priority for the Secret Service.

There was no indication of a threat from either the drone at the White House on Monday or the one in Germany in 2013.

Also read: A short recent history of small drones near the White House and Capitol