Security experts say prior knowledge is one of the only ways to prevent school shootings.
U.S. school districts have spent millions of dollars on metal detectors, security cameras and elaborate emergency-response plans since the 1999 massacre at Columbine High School, but almost nothing could have prevented Friday's massacre at a Connecticut elementary school, security experts say.
A 20-year-old heavily armed gunman opened fire Friday morning at Sandy Hook Elementary School in Newtown, Conn., killing 20 children and six adults before taking his own life, police said.
"The school could not have stopped it without prior knowledge that he was coming," said Bill Bond, a school safety specialist and former high school principal who had a deadly student shooting on his watch.
Unlike the shooting at Columbine in Colorado, where two students killed 12 other students and a teacher, the gunman in Newtown was not a student but an adult from the community.
"This is going to go down as a school shooting, but it's exactly what happened in Aurora," said Bond, referring to the mass shooting by a lone gunman at a movie theater in Aurora, Colo., in July that left 12 dead and 58 wounded.
"That was a movie theater, this was a school," he added.
Bond was the principal of Heath High School in West Paducah, Ky., where on Dec. 1, 1997, a student opened fire on a morning prayer circle, killing three.
He retired in 2000 and now works with other principals and students as a school safety specialist with the National Association of Secondary School Principals.
"We equate school shootings with students killing other students," Bond said.
It is still unclear what kind of security measures the school routinely took, but media reported that access was limited to visitors and doors were locked after 9:30 a.m. The shooting is believed to have happened soon after that time.
'SUBSTANTIAL' CHANGE AFTER COLUMBINE
Still, Bond said, there is no way for students and teachers to protect themselves when someone begins shooting with a weapon. He said even a security guard and locked doors could not have stopped the killer.
Without knowing the school's security system, Kenneth Trump, president of consulting firm National School Safety and Security Services, stresses that schools need to be "looking for these lone wolf actors."
"As a father, I would love to have a 100 percent guarantee that nothing bad will ever happen in my kids' school but as a rational school safety professional, I know nobody can give me a 100 percent guarantee on safety," Trump said.
But he has seen the danger drop at schools in the wake of Columbine. Dozens of shootings have been thwarted by students, teachers and principals sharing information and warning about students with weapons or plans to kill.
"There was a substantial amount of progress in the months and years after Columbine in prevention, security and emergency preparedness in our schools," said Trump. "A lot of those measures have become part of the school culture."
But he worries about maintaining that culture in the future because of budget constraints. As the risk of violence has increased because of the stress inflicted by the struggling economy, the time and funding allocated to these prevention and safety programs have declined.
"There has been an increased competition for time and money, and school safety has been losing on both accounts," said Trump.
Additional reporting by Stephanie Simon in Boston.
MSN News on Facebook and Twitter
Stay up to date on breaking news and current events.
Friend us on Facebook: www.facebook.com/news.msn
Follow us on Twitter: www.twitter.com/msnnews