Relying on building maps and "incredibly dramatic" live surveillance video, police outside the Colorado Planned Parenthood clinic that was under siege Friday were able to prevent the tragedy from becoming even deadlier, an official said.
The gunman — armed with an assault weapon — killed three people, including a police officer, and injured nine others at the Colorado Springs clinic.
But thanks to security cameras inside the center, which officers monitored from a command center, the toll wasn't higher, said Colorado Springs Mayor John Suthers.
"I have no doubt that but for the way they handled this, there could have been even more victims," Suthers, who watched the live video for hours along with the officers, told NBC News.
The five-hour standoff began at the Planned Parenthood building, where gunshots were first reported at 11:38 a.m. local time.
A SWAT team swept into the clinic after authorities tapped into the building's video surveillance system, monitoring the gunman's movements.
Their focus was two-fold: Catch the suspect, identified by police as 57-year-old Robert Dear, and free as many hostages as possible.
To accomplish the first goal, police passed along sketches of the building layout to the team inside, Suthers said.
"They were emailing sketches of the interior, the room set-up and that sort of thing," he said.
Police then watched as the gunman roamed the clinic. People had scrambled to all parts of the facility seeking safety — a file room, a consultation room — and as the gunman came toward them, "officers in the command center were communicating with them by cellphone, telling them to lay down on the floor," Suthers said.
Once it was determined that the assailant was no longer near them, the officers freed those hostages, he said. While Suthers didn't go into detail about the rescue operation, other officials told NBC News that several people escaped thanks in part to police personnel who rammed into the side of the building with a truck.
On the live surveillance video, "you could hear gunfire on occasion, you could hear them report an officer was down," Suthers said. "It was incredibly dramatic."
The officers could also hear the "calm professionalism" of their team inside, he said. "They just went about it in such a skillful and professional manner."
The security cameras were "very critical" to the operation, Suthers added, enabling officers to not just track the gunman's movements, but also his mood.
They watched as the 6-foot-2, 260-pound suspect coolly moved about, at one point hopping over a reception desk.
"You could see his demeanor, very calm, very deliberate," Suthers said.
For the last hour and 15 minutes or so of the tense situation, the gunman holed up in one part of the building. The SWAT team, using that information, prepared to barge through the doors and trap him.
But just before they began their confrontation, he surrendered, Suthers said.
"He started yelling, indicating that he wanted to give himself up," the mayor added.
Vicki Cowart, president and CEO of Planned Parenthood Rocky Mountains, said Saturday that the clinic's only security officer had left for the day before the attack began, reported NBC affiliate KOAA.
Speaking after a vigil in Colorado Springs for the victims, Cowart also said that all 15 employees at the Planned Parenthood survived the attack, and added that they kept patients safe and quiet during the shooting.
The officer who was killed was Garrett Swasey, 44, of the University of Colorado at Colorado Springs police department. Other officers said when they responded to the situation, they were "pinned down" by gunfire.
The two other slain hostages — both civilians — were not immediately identified.
Cecile Richards, president and CEO of Planned Parenthood Federation of America, thanked law enforcement in a statement Friday.
"Our hearts go out to the families and loved ones of the brave law enforcement officers who put themselves in harm's way in Colorado Springs," she said. "We are profoundly grateful for their heroism in helping to protect all women, men and young people as they access basic health care in this country."