A Las Vegas shooter's perch in a 32nd-floor hotel room overlooking 22,000 people jammed into a country music festival below is just the kind of nightmare scenario police dread in places where big crowds and high-rises mix.
From two broken-out windows of the Mandalay Bay Resort, Stephen Craig Paddock had an unobstructed view to rain rapid-fire bullets on the crowd, with few places for them to hide. Survivors of Sunday night's bloodbath that left 58 people dead and more than 500 wounded repeatedly compared it to shooting fish in a barrel.
In places like New York, Chicago and Austin, Texas, where big events are planned in city streets in the coming days, police sought to reassure jittery residents Tuesday of some of the precautions they are taking to prevent just such a scenario.
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New York City's police boss says that regularly includes sharpshooters with binoculars on rooftops scanning nearby building windows for potential threats, helicopters circling above with snipers of their own, and detectives making security sweeps of nearby hotels.
But he acknowledged there is only so much that can be done.
"We do understand," said NYPD Commissioner James O'Neill, "that no city or town in this country is completely immune to such unbridled hatred."
Added David Katz, CEO of Global Security Group, which conducts active-shooter training around the world: "The answer only really is, if there's a sniper, there's a counter-sniper."
But "you're not going to be able to deploy police units with sniper capabilities everywhere," Katz said. "There are, at some point, too many things going on, too many opportunities to stop them all. Unfortunately, if someone is intent on doing harm they will find a way to do it."
Chicago Mayor Rahm Emanuel, whose son will be among the 45,000 runners in the city's annual marathon Sunday, said emergency officials, including federal authorities, have conducted roughly a dozen workshops to talk through various scenarios and Chicago is prepared for "any eventuality."
"People don't just show up on marathon day and decide to run 26 miles. They train all year," Emanuel said. "That's also true of the Chicago police."
Despite assurances of a heavy police presence at this weekend's Austin City Limits music festival, expected to draw 75,000 people a day to the city's downtown, organizers were offering refunds to anyone uncomfortable with attending following the Las Vegas shooting.
Perhaps the most stark example of the crowd-building dynamic is in New York, where the city's 36,000-officer department regularly goes on high alert for such events as the New Year's Eve Times Square celebration, the Macy's Thanksgiving Day parade, Monday's Columbus Day parade, even some Yankees games.
For such events, the NYPD puts officers with body armor and high-powered weapons around the perimeter, sharpshooters on nearby rooftops to scan the windows of other buildings for threats, and cops with bullhorns on the streets instructing gawkers in nearby buildings to keep their windows closed.
They also have detectives ramp up security sweeps at hotels, particularly ahead of the holiday season. And the NYPD has a program to train thousands of private businesses and employees, from housekeeping staff to security, on how to spot explosives or tell a golf bag from a gun case.
David C. Kelly, associate managing director K2 Intelligence and the former assistant commissioner for counterterrorism at the NYPD, said the shooting forces private security and law enforcement alike to give more regular events treatment usually reserved for special occasions like a president or a pope's visit.
"It's a big ask, but maybe that's what needs to be done now," Kelly said. "It's forcing law enforcement to look at this in three dimensions, the car in the crowd, the bomb in the backpack, now the assault from the air."