Jackie L. Burks, center, is pictured in the District’s new homeland security hub on July 4 as she initiates the department’s “Safe Haven” protocol with lightning threatening crowds on the Mall. (Sarah L. Voisin/The Washington Post)

It wasn’t a drill. It was happening, and now: Open the doors, suspend the bag checks, turn off the magnetometers, let everyone inside.

The plan had been developed to shelter masses gathered on the Mall in the case of an emergency weather situation, but the evacuation ahead of Saturday’s Independence Day celebration also signaled what might happen in the face of a terror threat: “Operation Safe Haven.”

From loudspeakers, U.S. Park Police officers began clearing early Fourth of July revelers from the lawn near the Washington Monument. A dangerous thunderstorm was fast approaching.

For the anxiety the announcement induced for visitors there, the plan unfolded with matter-of-fact precision inside a hidden command center four miles away. The event’s chief law enforcement officer picked up a microphone and initiated the protocol with a single sentence. There was no further explanation needed.

A room filled with representatives from the police and fire departments, FBI agents, federal marshals and even liaisons to the National Gallery of Art, Smithsonian Institution and the U.S. Holocaust Memorial Museum began implementing well-practiced plans.

Emergency evacuation routes as well as a news station broadcasting a heightened security alert are displayed on a wall of D.C.’s new Mutli Agency Coordination Center on July 4, 2015. (Sarah L. Voisin/The Washington Post)

A little more than a year after it was activated as the city’s Multi-Agency Coordination Center — or MACC, in homeland security lingo — the nondescript brick building in Southeast Washington has brought a new level of sophistication to managing iconic events in the nation’s capital.

The technology inside the District’s new base is also approaching what people might expect after watching a television show starring Kiefer Sutherland.

There is a wall of projection screens and access to footage from thousands of cameras and live feeds, including from a helicopter that might be circling the Mall. There’s a running list of locations and conditions of every ambulance patient citywide, as well as the number entering each medical tent and cooling bus set up along the Mall. At every desktop, there is even an app with the GPS coordinates of every trash bin, portable toilet and chain-link fence erected from the U.S. Capitol to the Lincoln Memorial.

With a single click, every camera at a chosen location can be pulled up for review.

As news programs on Saturday blared warnings of increased threats of terrorism, there was little question about why the facility was built.

With staffers from agencies including the city’s Department of Transportation and trash collection, the Coast Guard and the Secret Service in one room, the operation is a rare picture of coordination between the District and the dozens of federal law enforcement agencies that call it home.

A Washington Post reporter observed the homeland security operation Saturday, and at 3:45 p.m. a glimpse of the extensive planning came into view.

The emergency operations center’s manager announced an impromptu briefing on the weather. A representative from the National Weather Service took the microphone and said that a band of storms with lightning and wind gusts up to 30 mph was moving through the region. One cell was taking dead aim at downtown.

Jackie L. Burks, deputy chief of the U.S. Park Police — the lead agency, along with the National Park Service, because the event was on the Mall — listened intently. Within seconds, she stood up and declared that the two federal agencies would initiate “Safe Haven.”

The plan began several years ago with an informal system of asking Mall museums to speed sightseers inside during inclement weather.

In the past three years, museums and security officials for more than a dozen buildings there have agreed to suspend security procedures and allow people to flood into common areas of museums and hallways of federal agencies in the event of a declared emergency.

At the Natural History and American History museums, security guards suspended their screenings and poncho-clad crowds packed into the entrances, stairways and hallways by 4:30 p.m.

Brian Baker, chief of staff for the D.C. Homeland Security and Emergency Management Agency, said the agency has run simulations for using the procedure to limit fallout in the instance of a dirty-bomb nuclear attack.

At a major event such as the annual summer concert and fireworks show, officials said, perhaps 200,000 people could fit into buildings that were opened Saturday. During a presidential inauguration, when as many as a million people descend on the Mall, even more downtown buildings could open to take in several times that figure.

Saturday marked the second July 4 that the command center was fully activated. In between, it served as the homeland security hub for the State of the Union address, the U.S.-Africa Leaders Summit and every other large-scale event on the Mall, including last year’s Concert for Valor on Veterans Day.

The activation wrapped up early Sunday morning after what amounted to a 19-hour practice run for the more than 160 representatives from law-enforcement and other D.C. and federal agencies who worked from the facility.

After 10 p.m. the previous night, with crowds peacefully leaving the Mall, the National Park Service took to Twitter to declare a successful Fourth. The hour-long Safe Haven exercise had made up the center’s most tense moments of the day.

“What’s great is that we practice so often that we really do get to know each other,” said Nicole Chapple, a spokeswomen for the District’s homeland security department.

The next time the command center is scheduled to activate is for the pope’s visit in the fall.