Secret Service holds first full-scale training exercise at the White House

— -- Just before dawn Friday, the U.S. Secret Service held its first full-scale training exercise involving potential threats at the White House.

In one scenario, agents responded to an intruder who breached the White House fence, then an attack dog took him down.

There were other emergency situations, including medical, others classified for security reasons. Today’s training was developed over the course of many months and involved around 100 agents, officers and other personnel. It took place across the entire White House complex.

Agents usually train at the James J. Rowley Training Center in Laurel, Maryland, where there is a K-9 training facility, a driving course, empty buildings for attack scenarios and more.

But officials say nothing approximates the scale and the complexity of the White House.

The new Secret Service Director Randolph Alles -- a former Marine general -- says he is employing his past experience: we train like we fight.

"The Secret Service is bound to the unique no-fail mission of protecting the White House,” said Alles in a statement. “The worldwide threat environment is real. These threats require us to constantly train so that we can evolve with the trends and tactics of our adversaries.”

The protective agency decided that the current threat environment called for something unprecedented -- a full scale training exercise with the White House up and operating.

“We want to always train in the most realistic environment possible,” said Kimberly Cheatle, U.S. Secret Service Special Agent in Charge of training, in an interview with ABC News.

She said that it offered the opportunity to test command and control, operational readiness, communications and myriad other scenarios

“We always want to test for the ‘what if’ and all the possibilities that are out there,” she said.

When asked why this had never been done before, Cheatle said, “There are obviously precautions we wanted to take on an operational facility and those considerations had to be taken into account when we were plotting out this particular exercise.”

Numerous intruders have breached and attempted to breach the White House perimeter over the past few years. And there are new potential threats like weaponized drones, which the Department of Homeland Security (DHS) warned about in its updated security bulletin this year.

"The Secret Service is constantly evaluating what’s happening in the environment, in the world, in the threat environment, so we’re constantly conducting those assessments and then adjusting from there," said Cheatle.

The goal of today’s training was to better prepare agents who protect the president to deal with emerging threats like drones and avoid the missteps of the past.

The White House exercise took place before President Trump left for the Christmas holiday, though the Secret Service would not disclose the itinerary of the President.

The agency said it received the full cooperation of the administration during the planning and coordination of this exercise.

While this was the first dynamic exercise on the White House grounds, the Secret Service has conducted four White House complex themed table-top exercises, as well as an active shooter training at the White House since May.

The Secret Service said it is constantly looking at ways to enhance and modernize security. For instance, construction on an updated fence -- designed to better keep out jumpers -- is expected to begin in the spring of 2018.