- First responders reacted correctly in clearing the area, security experts say
- Moving people away from the blast minimized chaos and preserved evidence
- Nothing can fully safeguard major U.S. events like the Boston Marathon
- President Obama urges Americans to "speak up" about anything suspicious
When the first bomb went off near the finish line of the Boston Marathon, emergency personnel knew to expect another.
It occurred just 12 seconds later, and the first responders already present for Monday's world-famous race that drew tens of thousands of visitors to the city immediately started moving people away from what had been a celebratory scene.
In clearing the area so quickly, authorities acted on lessons learned from past terrorist attacks such as the Oklahoma City bombing in 1995, the Atlanta Olympics bombing in 1996 and the September 11, 2001, attacks, according to security experts.
Major public events such as a marathon in a nation that prizes personal freedom can never be made perfectly safe, said Juliette Kayyem, a CNN contributor and former homeland security official.
What authorities can do is establish effective responses and procedures to safeguard as many people as possible and protect the integrity of evidence at the scene, she noted. That means making sure first responders know what to do, she added.
"Look, we're an open society. Marathons are open events. They are hard to secure," Kayyem argued when asked about preventing such attacks. "The better way to look at it, I think, is did we respond better? I think the answer is yes."
Video footage showed the magnitude of the challenge, with runners approaching the end of the marathon where race workers and well-wishers awaited them.
Seconds after the explosions, some first responders and onlookers pore into the smoke and wreckage while others begin helping exhausted racers continue moving past the carnage.
Hundreds of other runners were diverted from the marathon course further away, preventing them from getting close to the bombing scene.
Kayyem called the moves "brilliant," saying they achieved two critical objectives.
"One was the chaos was minimized," she said. " ... And then secondly, it preserved the crime scene, which is going to be key for the FBI investigation. Those are lessons learned out of 9/11."
Rep. William Keating, D-Massachusetts, noted that almost 2,000 law enforcement personnel took part in a planning exercise for the possibility of such an attack last November.
"They're prepared for this," Keating, a member of the House Homeland Security Committee, told MSNBC. "Are you ever prepared for everything? No."
The committee will examine what happened in Boston "at the appropriate time," he said, adding: "We are going to be peeling back, looking at this evidence, seeing what happened, seeing what we can do to improve. With terrorists today, it's a moving ball."
To James Carafano, a security expert who is vice president of Heritage Foundation, the only way to eliminate the risk of terrorism at major events such as the Boston Marathon is to stop holding them.
"Having said that, there are basic public safety procedures for events like this, which are well-established," Carafano told CNN. "We've used them. We've learned from everything from the '96 Olympic bombings to what you should do."
Police alerted to a suspicious device already were clearing the crowded Centennial Olympic Park in Atlanta when it exploded almost 17 years ago, killing one person and wounding more than 100. Within minutes, widening lines of police officers sealed off the area by forcing back bystanders and journalists.
Authorities eventually linked the bomb to others set off by Eric Robert Rudolph, who was sentenced to life in prison.
Now, they seek a bomb signature from Monday's blasts that will help them figure out who planted the devices.
Fran Townsend, CNN's national security analyst, said the modus operandi of terrorist bombers -- whether foreign or domestic -- usually involves multiple devices because one may not go off.
To Michael Sullivan, the acting head of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, the two Boston explosions could have sought to cause people fleeing the first blast to move toward the second one.
The number of terrorist bombings in the United States has decreased dramatically in the 12 years since the 9/11 attacks, due to the nation's heightened security.
"We have stopped many of the terrorist plots and attempts from the past through good intelligence," Rep. Michael McCaul, R-Texas, who chairs the House Homeland Security Committee, told CNN. "In this case, we really didn't have any. There wasn't a whole lot of chatter."
One advantage for authorities in an attack on a large event is the number of emergency personnel and others already present at the scene.
At the marathon finish line, teams of doctors and nurses on hand to help runners completing the race were able to offer immediate first aid to bombing victims.
"You couldn't have had a larger group of physicians in one spot," said Dr. Albert Pendleton, an orthopedic surgeon who was part of the medial contingent for the marathon.
"I helped some people get back to the medical tent and then they told all the doctors to go back out there," Pendleton told CNN. "And so we went out there and there was probably four people on every single person who was down in that area, you know, starting IVs, you know, getting back boards in there, getting gurneys and getting (victims) shuttled into the medical tent."
President Barack Obama praised such efforts on Tuesday, saying in remarks that "the American people refuse to be terrorized."
"What the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love," he said, citing "the first responders who ran into the chaos to save lives."
The president also exhorted citizens to remain vigilant, saying that "this is a good time for all of us to remember that we all have a part to play in alerting authorities."
"If you see something suspicious," he added, "speak up."