For decades, law enforcement has tried to keep one step ahead of terrorists who target large groups and venues.
Authorities have beefed up security, employed metal detectors and chemical weapons detectors, erected barriers and more recently tried to better control traffic choke points where attackers strike as people tried to flee.
But the Las Vegas mass shooting underscored how difficult it is to keep large groups safe despite precautions. The Route 91 Harvest country music festival included plenty of security and many of what today are normal precautions.
But a gunman opened fire on concertgoers from the 32nd floor of a nearby resort, leaving at least 58 dead and more than 500 injured. Authorities said they believe Stephen Paddock, 64, acted alone and did not have ties to overseas terrorist groups.
"It is so so challenging when you have shooter in a very very high position," said retired LAPD Cmdr. Rick Webb, an expert on active shooter scenarios. "It was very well thought on his parts, it is a horrible tragedy."
"This is the new normal," added Brian Levin, a counter-terrorism expert and professor at Cal State San Bernardino. "We are now seeing lone wolves being able to create war at our entertainment venues."
Levin and others said some law enforcement agencies have envisioned this type of scenario.
Both New York and Los Angeles police have used a blanket of snipers on roof tops to target any potential high-perched shooter.
"It is very hard to secure any large crowd from elevated positions in Las Vegas." Levin said.
The Los Angeles Police Department's Metro Division has used a small platoon of snipers, some armed with .50-caliber rifles capable of picking off a gunman at long range from rooftops. It is an approach long used by the Secret Service to protect dignitaries.
Experts say that after the Las Vegas mass shooting, there will be a reassessment nationwide of how to protect large open space gatherings.
Levin said government agencies may now consider using armed drones to disarm suspects.
Terrorists tend to target areas where large crowds gather. Attacks in recent years in Berlin and in Paris and Nice, France, have prompted Southern California law enforcement officials to review security measure at train stations and venues such as Staples Center and Dodger Stadium.
But the question of how best to protect crowds dates back to at least the 9/11 attacks. Security experts have long expressed concerns about a possible terrorism incident at Los Angeles International Airport, and worry in particular about the outside of the terminals where passengers sometimes must line up. LAX has been working to reduce lines forming outside.
Last year's deadly terrorist explosion at an Ariana Grande concert in Manchester, England, heightened concerns about how large venues are secured.
British counter-terrorism investigators think the attack may have been the work of a suicide bomber who entered a crowded area outside the performance space where attendees were streaming out of the concert.
Experts said at the time that the attack showed the need for more vigilance in areas outside those security zones, such as transportation centers, walkways and parking lots.