Operating drone aircraft in America’s national parks has been against the law since National Park Service director Jonathan Jarvis signed a policy memorandum establishing the ban on June 20.

The policy memorandum directed park superintendents across the U.S. to prohibit launching, landing or operating “unmanned aircraft” on land and water administered by the National Park Service.

Since then, Yellowstone National Park rangers have cited three tourists for illegally flying a drone in the park. Two were from Europe.

According to Yellowstone Park officials, Donald Criswell of Molalla, Ore., flew an unmanned aircraft over Midway Geyser Basin and near bison on Aug. 19.

Park rangers charged Theodorus Van Vliet of the Netherlands in early August after his drone allegedly crashed into Grand Prismatic Spring.

And Andreas Meissner of Germany is accused of flying a drone that crashed into Yellowstone Lake on July 17. He was charged later in August.

Rangers in Grand Teton National Park cited a man in late June after his drone crashed into a tree while he was trying to get aerial photos of wildlife.

The Park Service’s ban came after several incidents involving drones occurred. In September 2013, a drone flew above evening visitors seated in the Mount Rushmore National Memorial Amphitheater. Concerned for visitors’ safety, rangers seized the unmanned aircraft.

In April this year, a drone flew back and forth above visitors watching the sunset at Grand Canyon National Park. The unmanned aircraft then crashed into the canyon.

Later in April, volunteers at Zion National Park witnessed a drone flying close to a herd of bighorn sheep, reportedly separating adults from young animals.

Glacier Park rangers have not cited anyone for operating a drone inside the Park without the superintendent’s written approval, spokeswoman Denise Germann said.