When the radio call announcing “shots fired” went out shortly after 8:20 a.m. on Sept. 16, Capitol Police Officer Melissa Marshall was among the first to deploy as part of enhanced security operations around the Capitol.

Marshall, who started her shift at the South Barricade at 7 a.m. that day, had been headed for her morning break but quickly changed her route, grabbed her M4 rifle and a few extra magazines of ammunition and headed back to her post.

Marshall, a member of the elite Capitol Police First Responder Unit, and the other officers at her post listened to Capitol Police and Metropolitan Police Department radio updates on the mass shooting under way at the Navy Yard, trying to grab information that might help assess potential threats to the Capitol.

Another elite Capitol Police unit, the Containment and Emergency Response Team, was heading to the scene and was allegedly told to stand down when it arrived, a situation currently under review by a Capitol Police Board team.

“We still had to keep things running smoothly here,” Marshall said on Wednesday, reflecting on the experience. “Even though we were on high alert, Congress still has a job to do and we need to make sure that we’re protecting them.”

Another member of the First Responder Unit, Officer Paul Nebel, was also on the 7 a.m. to 3 p.m. shift that day and was stationed on the lawn of the East Front of the Capitol with an M4 strapped to his chest. He was part of the beefed-up operations set up to protect everyone under the Dome and in its immediate vicinity from the threat of an active shooter.

While scanning his surroundings for threats, Nebel was also fielding questions from tourists curious about the presence of armored vehicles and officers in bulletproof vests and helmets.

“I don’t tapdance around any question,” he said. “I feel it’s better to be honest and upfront with these guys and just let them know what’s going on, and I try to be polite and professional because that, too, is part of my job. You’ve got to learn how to polish those skills and use them effectively, just as effective as that M4.”

The First Responder Unit is an elite, close-knit group of more than 150 officers charged with protecting the members, staffers, journalists, visitors and thousands of tourists crisscrossing the grounds of the Capitol each day. CQ Roll Call was given the opportunity to talk to four of the unit’s officers about how they ensure the security of the Capitol and the people inside.

When a threat emerges, “it should start and end with us,” Nebel said. “That’s our job, to make sure the threat never gets in the building.”

The unit was created in the summer of 1985, in part a response to the explosion of a bomb outside the Senate chamber in 1983.

A fit, young group of officers, among them eventual Assistant Chief Tom Reynolds, was selected to be part of an elite new guard. They would be outfitted with dress uniforms and responsible for responding to emergency situations and threats to the Capitol.

Reynolds remembers wondering, “What have I gotten myself into?” on his second day, when a Greenpeace demonstrator distracted his fellow officers by climbing a crane on the West Front of the Capitol.

The unit also had to “line up pedestrian visitors on the plaza to make sure they stayed safe from congressional traffic” and receive dignitaries, Reynolds said. He recalled receiving President Ronald Reagan from a helicopter after a trip to Russia.

Today, the more modernized unit still prides itself on coordinating with the Secret Service for presidential visits — State of the Union addresses and inaugurations are among their favorite days on the job — but also confronts post-9/11 threats to the Capitol, such as terrorism and chemical weapons attacks.

Officer Ryan McCamley, who has been a member of the First Responder Unit since 2006, has FBI training in crisis negotiation and recently volunteered for training with the Advanced Law Enforcement Response Team. ALERT helped respond to the 2001 anthrax attack and the ricin attacks in June; McCamley’s training prepared him to help collect evidence, support perimeters and decontaminate buildings.

“To me, it kind of combines the skills of a firefighter and a police officer, so I thought it was kind of interesting as a way to get new training,” he said. “Not a lot of law enforcement agencies have their own hazmat team. Also, not a lot of law enforcement agencies have to deal with the same kind of chemical attacks that the Capitol and the Hill have had since 2001.”

In addition to being alert for the first signs of trouble on the Capitol square, first responders must stay sharp for the thousands of tourists wandering around the grounds full of questions.

“D.C. is the only place in the world where people will approach a police officer from behind, without introducing themselves, and just start asking questions,” Officer Ryan Robertson said.

Robertson has taken it on himself to develop a mental library of answers, so when tourists ask — “Who’s on top of the Capitol Building? Who built it? Where’s the cornerstone? How tall is the Rotunda?” — he can answer quickly.

It’s made him a sought-after tour guide. During Police Week each May, he gives “dozens and dozens” of after-hours tours to officers visiting from around the nation, each as long as three hours. He’s also called on to show federal law enforcement officials and military command leaders the ins and outs of the storied marble halls.

“When I was a rookie, I’d get a break and I’d scour this building — get into every room, everywhere I could. It’s our building; we need to know it,” Robertson said.

Physical discipline is essential to maintaining the stamina required to endure eight hours under the baking mid-July sun or in the blistering cold December winds that stir the branches of the Capitol Christmas tree on the West Lawn.

“There are not too many days that we stand out there and it’s 70 and sunny,” Nebel said. “You’re either going to be sweating or you’re going to be freezing.” He’s committed himself to a daily 3 a.m. wake-up call to squeeze in a two-hour workout before the duties of the first shift. It’s not the easiest schedule for a married man with two sons, ages 10 and 14.

“I commit to it and take it seriously because those guys are looking to me as an example of doing the right thing even when no one is watching,” he said.

McCamley’s commitment to the physical demands of the job led him to krav maga, a self-defense system based on boxing, wrestling and street-fighting that was brought to the United States from Israel in the mid-1980s. He began taking classes in 2006 at a center in Columbia, Md., and now trains civilians on the side and conducts offsite law enforcement trainings for local, state and FBI officers. He also leads pre-deployment courses for CIA, National Security Agency and special forces groups.

Robertson sums up the highly motivated group of officers like this: “99 percent of us are a type-A personality. … We’re the first protection of the U.S. Capitol. We’re the first ones you’re going to meet when you come to the square and this square is ours. Anything that happens inside, we’re going to deal with.”