It was an amazing sight: A gunman on the loose at the Washington Navy Yard, and a U.S. Park Police helicopter hovering into range, on purpose.

"We were responding to an incident where the report was subjects who were armed with 23-caliber rifles, very high powered," Sgt. Kenneth Burchell, a pilot with the Park Police, told ABC News. "Very aware going into that environment, [we] wanted to get to people as quickly as we could."

The helicopter hovered just above the building tops, while lowering a rescue basket to tactical police below in order to pull four people to safety, one of them wounded badly.

"If you can imagine, got something on a string, if we pull it up, the basket will begin to oscillate," Park Police rescue technician Sgt. Dave Tolson said. "Not in a very good position to stop that oscillation, keep centered as much as possible to stop that swinging from taking place. [We] don't want to swing them out of the basket."

U.S. Park Police Officer Michael Abate said the urgency to do something so risky was to save lives.

"We had people on the roof that needed to be evacuated; main priority," he said.

They didn't know much about the killer at the time, just that he was on the attack and might have one or two accomplices.

So Mike Abate leaned out of the helicopter, an M-16 on his shoulder providing cover for his partners, the pilots and the man lowering the basket.

"A rifle for a rescue? This would be a first, putting them both together is a first," Abate said. "I knew there was an active shooter, initially heard there might be two and there was a lot of chatter [on the radio]."

Just 30 feet below their rotors, there turned out to be just one gunman. Aaron Alexis was on a shooting rampage, which, according to FBI officials, lasted between 30 and 60 minutes, taking a dozen lives.

Since the Monday attack, details have emerged on the path the former Navy reservist took, entering the front door of building 197 with his valid security pass and carrying a duffle-type bag. He hustled directly to a bathroom on the fourth floor, walking out into the hallway shooting, and continuing his rampage down to the third floor. Standing on a catwalk overlooking the atrium and shooting down into the food court cafeteria. Finally, down the stairwell to the first floor, killing a security guard, before taking his handgun.

Meanwhile, on the roof, sergeants Burchell and Tolson of Park Police executed the most dangerous kind of helicopter rescue, lowering a hoist to the rooftop of building 197 to help an injured woman in desperate need of immediate medical attention.

"Hoist operations are one of the most challenging, if not the most challenging," Burchell said. "It puts the most amount of strain on the aircraft. Requires the greatest level of crew coordination."

At risk of becoming targets of the active shooter, the team hovered over the roof and had just minutes to make their move.

"While I'm lowering this basket, I'm doing so from a moving helicopter and winds change," Tolson said. "So it is very important that we maintain our position over the victim, because if we were to move and that pulled the basket and she wasn't properly loaded in there, we might take ourselves out of position or worse, still, create an unsafe situation for the victim."

They reached the still conscience woman, who has a serious shoulder injury and was bleeding badly, so they rushed her to MedStar hospital.

"She did describe the suspect for us and her where about as she last knew them," Tolson said. "That's why I commented she was incredibly composed and very brave."

With impressive team work, they are able to airlift all four people to safety.

"We are definitely not heroes but we saw a lot of heroes out there," Burchell said. "We are just guys. We just did our part."

(Image Credit: Jacquelyn Martin/AP Photo)