The mass shooting that killed at least 59 people and wounded hundreds of others in Las Vegas Sunday night was a security expert’s worst nightmare: tens of thousands of people packed into an outdoor venue close to a tall structure — the Mandalay Bay Resort and Casino — that provided a high perch for a gunman.

In a situation like that, the protocol for an active-shooter situation goes right out the door, said Dan Bongino, a former Secret Service agent and officer in the New York Police Department.

“This is the nightmare scenario when it comes to soft-target tactical assaults,” Bongino told Fox News. “You have a shooter with an almost unlimited supply of ammunition, in this case from an elevated platform.… Everything you have been told about active-shooter scenarios goes out the window. You have been told to duck and cover, seek cover and concealment, get low, get small.

“And those are all good pointers. But when someone is shooting from an elevation, they are shooting over the objects.”

The Las Vegas shooting will force a change in security measures for outdoor concerts, said Gregory Boles, associated managing director of security risk management at Kroll Associates, a New York-based risk consulting firm.

“They will probably take into account any high ground above the event,” he said. “You are very vulnerable when there are structures that are in the immediate vicinity, that are considered high ground, where someone could, with a rifle, shoot down at the crowd.”

But it’s very hard to predict an attack like this, Boles continued.

“It’s hard to believe you would be able to prevent something like this, other than at the hotel looking for anomalies. When someone checks in, and they check in with a bag, that is normal. But when you have a single person coming in with several bags, that is an anomaly, or someone who does not fit [the] regular pattern of normal behavior.”

There are countless open-air concert venues across the country —Stagecoach, The Greek Theater, The Hollywood Bowl, Red Rocks Amphitheatre, Jones Beach theatre. Each creates its own security problems, and they are different from those of an indoor venue.

The problem with an open-air event, which we have seen from this, is you don't have a roof — and a roof blocks the line of sight from any type of elevated platform attack or sniper-style attack,” Bongino said. “There is no roof, so therefore people can see down and engage targets on the ground.”

Jason Porter, a vice president at the Pinkerton private security guard and detective agency, aid a situation like Sunday’s shooting is hard to predict.

“This is a situation in which I don’t think anybody anticipated for, and it would have been very difficult to anticipate for,” he said. “I think moving forward, this is definitely going to be on the forefront of people’s minds, and it will prompt a conversation to adjust policies and procedures.”

Bongino said the chance of an attack like Sunday’s is “low, but not zero.”

“My biggest fear is that it will inspire copycat style attacks,” he said. “I think, going forward, if you don't have a really robust counter-sniper program in [a] big city police department and you are handling big venues like this often, it’s time to re-evaluate that approach."