As Aaron Alexis stalked through Building 197 of the Navy Yard last September, killing a dozen employees with a modified shotgun and a semiautomatic pistol, 170 law enforcement officers, including 57 D.C. police officers, barreled into the building determined to stop his rampage.

Within six minutes after Alexis began firing, 10 of his 12 victims were mortally wounded, but the gunfire continued for more than an hour.

Officers formed small, ad hoc shooting teams as they poured into a building with an open atrium that provided little cover and a maze of cubicles to hide a gunman. Police improvised on the fly: Overturned furniture became makeshift barricades and a key card snatched from a fallen security officer was used to access secure areas. Officers propped open doors so backup units could flood in.

During eight firefights with Alexis, no civilians were hit, but two D.C. officers were shot by the gunman.

Officer Scott Williams suffered wounds in both legs and was pulled from the fray by colleagues. Officer Dorian DeSantis’s protective vest stopped rounds that struck his chest, and he was able to return fire, fatally wounding Alexis to end the threat.

“It was really amazing to watch the video of their actions when they went into that building,” said D.C. Police Chief Cathy L. Lanier. “As soon as responding officers entered the building, they could hear the gunfire.

“Every police officer that entered that building was unequivocally amazing.”

Lanier revealed new details of the Navy Yard massacre Thursday night at the department’s 14th annual awards ceremony, praising “the dedication, courage and professionalism of every officer” called to the Navy Yard the morning of Sept. 16. She awarded the department’s highest honors to many who responded that day.

The chief described how surveillance video shows wave after wave of officers from multiple agencies combining into teams to search the huge building.

Richard “Mike” Ridgell, a security guard and former Maryland state trooper, was killed by Alexis near the building entrance. Officers who came upon his body took his key card so they could access secure areas, police said. Officers pulled out pieces of furniture to provide cover in open areas.

“It’s not something we train to do; they had to actively think about doing it under fire,” Lanier said.

Lanier said 170 police officers engaged in eight separate gun battles with Alexis.

“I don’t yet know how many rounds were fired in the building. But the fact that they were able to take [Alexis] out of the equation in one hour and nine minutes, in a building that was 630,000 square feet that he could have hid in for weeks, is amazing,” Lanier told the crowd of hundreds of officers and their families.

Lanier awarded the Chief of Police Medal of Merit to a U.S. Park Police pilot, Sgt. Kenneth Burchell, along with rescue technicians Sgt. David Tolson and Officer Michael Abate, who flew in police helicopter Eagle 1 to rescue an injured victim from a rooftop.

During the chaotic first minutes of the shooting, a woman who was wounded in the shoulder was helped to the roof by colleagues, who wrote a note pleading for help and dropped it to the ground. A D.C. police officer recovered the plea, and the helicopter was sent in. The woman survived.

Special Agents Brian Kelley and Ed Martin of the Naval Criminal Investigative Service, who joined D.C. police officers Williams and Emmanuel Smith in the search for Alexis in a corridor lined with cubicles, also received decorations. At one point, Alexis emerged from hiding and shot and Williams in both legs, police officials said. Smith provided cover fire as Kelley and Martin pulled the wounded officer to safety and carried him down three flights of stairs.

Park Police officers Andrew Wong and Carl Hiott received Medals of Merit for joining DeSantis in the firefight that ultimately killed Alexis. Police officials said the three-man team entered the cubicle area where Alexis was hiding.

Wong covered hallways leading to the cubicles, while DeSantis and Hiott rounded a partition seeking the gunman. Alexis emerged a few feet from DeSantis and fired, hitting him in his protective vest, police said. Both officers returned fire.

When it came time to accept his awards, DeSantis, a 24-year veteran of the department, shuffled nervously before stepping to the podium. “I’ve got two families here tonight, my MPD family and my own family,” he said. “This is a team effort really, all the way across the board.”

In brief remarks, Williams said “it means a lot” to him that so many officers and his family visited him in the hospital as his legs healed.

As Lanier lauded acts of courage, fellow officers and civilians stood to applaud. Many guests treated the heroes of that day like celebrities, particularly Williams and DeSantis, who were asked to pose for photos and to accept hugs and handshakes.

Both officers were awarded the Medal of Valor, Medal of Honor and the Blue Badge Medal.

“The responders at the Navy Yard did much more heroic and amazing things than the public will ever know, but we should all know that what they did that day saved countless lives,” Lanier said. “We owe them a special thank-you.”

Get updates on your area delivered via e-mail