But experts are divided over whether police should have immediately gone into the school and confronted Lanza, or waited -- as they did -- outside for six minutes.
In the bloody last seconds of the murder spree, investigators said Newtown police cars arrived and parked more than a football field away.
The long-awaited release Wednesday of recorded calls for assistance and the background sounds of rifle shots that killed 20 children and six adults may provoke more questions than provide answers on the important issue of the police response.
During the 47 seconds in which Lanza fired at least nine shots from his semi-automatic Bushmaster XM-15 assault-style rifle and a Glock 10mm handgun, first responders were setting up outside the school on Dickenson Drive.
"Since Columbine, the message is go after the shooter as quickly as possible," said Brookfield Police Chief Robin Montgomery, speaking on the evolution of tactics in active-shooter incidents.
In a suburban department, such as Brookfield or Newtown, the initial responders will be patrol officers.
"You don't have time to gather the special-ops folks," said Montgomery, who hadn't listened to the recordings and declined to speak directly about the Sandy Hook response.
But in mass slayings, the only person who knows what's going on is the shooter, said James Strillacci, the former West Hartford police chief and longtime Capitol lobbyist for the Connecticut Police Chiefs Association.
"Responding officers try to get in as quickly as they can," said Strillacci, noting that terrain and the state of the school were major factors facing police. He said that 47 seconds probably wasn't important.
"You'd have to be very lucky and efficient or both," Strillacci said. "It's a pretty tough situation. The male dispatcher did pretty good triage, getting as much information as he could."
Retired New Haven police officer Vincent Riccio, who operates the Security Academy of Connecticut, hadn't yet listened to the 911 tapes, but said he thought six minutes was too long for the cops to wait to enter the school. He thought they might have gotten distracted by people outside the building, who turned out to be two women and a parent with a cellphone.
"Who would you rather see inside, a police officer with a gun or a teacher without a gun?" Riccio said. "I'm not saying they're bad cops ... they are brave enough to put on a uniform and go to the scene. I'm saying they made a bad mistake."
"It's all about tactics," he said. "They have to move toward the shooter. If they were there for 45 seconds, they should have gone into a sprint and moved from cover to cover toward the shooter. To give them a pass because we know in hindsight (Lanza) was dead doesn't matter."
"You can't lose focus," Riccio said. "According to FBI statistics, 97 percent of active shooters are lone shooters. I believe they lost their focus."
Another retired police officer, who did not want to be identified but who has been involved in shooting cases, said police officers are taught never to park near the front of a building or approach directly from behind a closed door.
"If they were in a parking lot 360 feet away from the entrance, that's acceptable," he said.
He didn't think police could have accomplished much in 47 seconds.
"So many things had to be going through their minds," he said. "What if the first responder goes in and is taken out? What does that accomplish?"
Robert Paquette, a former FBI supervisor who worked with SWAT teams and was the Danbury police chief until 2005, agreed.
"It's difficult to judge the officers not knowing everything that was going on," Paquette said." It takes 47 seconds to dial a phone number."
He declined to judge the officers' actions.
"You don't know what instructions they received," he said. "You don't go running into a situation unless you have a scintilla of what to expect. I don't think the officers knew anything except shots were being fired. I can't imagine trying to make a decision based on all the chaos going on. There were talks of a second shooter."
"If you know what you're facing and have the gunman in your sight, then you've got to approach and take him out," Paquette said.
On the 911 recordings, Rick Thorne, acting head custodian that fatal morning, pleaded for police to arrive and stop Lanza's murderous rampage.
"I keep hearing shooting...I keep hearing popping," Thorne told police dispatchers at 9:37:26, shortly before police finally reached him, demanding to know who he was.
At 9:39, just as Newtown Police Officer Michael McGowan arrived on Crestwood Drive behind the school, Thorne told dispatcher Bob Nute that a gym teacher reported seeing two shadows going past the gym. Nute questioned whether the shadows were inside or outside. Thorne replied they were outside.
At 9:39:35, McGowan, said "We got him coming at you." He was talking about the parent with the cellphone.
On the tape, Thorne finally sees cops at 9:45:02. According to a recent summary of the shooting by Danbury State's Attorney Stephen J. Sedensky III, however, they entered about 12 seconds earlier.
Thorne can then be heard on the tape identifying himself by shouting "Custodian, Custodian."
The release of the six recordings -- after a months-long battle -- first before the state Freedom of Information Commission and later in state Superior Court, caused the information to be delayed until just before the first anniversary of the slaughter on Dec. 14.
W. John Thomas, professor of law at the Quinnipiac University School of Law, said that the release of the 911 tapes so close to the anniversary hurts the survivors and families of the dead, whom Sedensky was trying to protect by suppressing the recordings.
"I don't think the state's attorney has done any service to Newtown or the state for delaying the release," Thomas said in a phone interview after listening to the recordings. "It's harrowing information but the judgment of the Connecticut General Assembly is that it's the kind of information that can be released. Now, the families have to relive the tragedy a year later and it might have been better to comply with the law first. They should have been released earlier."
Gov. Dannel P. Malloy said Wednesday night that he had listened to some of the Sandy Hook 911 tapes and read transcripts of others.
The governor, speaking with reporters at the University of New Haven where he thanked first responders for thwarting a possible shooting spree, said he didn't think it was necessary to listen to the tapes to underscore the tragedy of the shooting.
Malloy said he was struck by the heroic actions of the Sandy Hook school staff.
"There's a lot of evil in this world," Malloy said. "Those who are evil are vastly outnumbered by the goodness that's present and the extraordinary efforts that individuals make to keep one another safe. You can't read or listen to a tape like that or understand what these folks go through on a daily basis without having great admiration for their humanity," he said, alluding to the police officers flanking him.
Staff writer Wesley Duplantier contributed to this report.