Last holiday season, New York-based private investigator and security consultant Bill Stanton went to see Radio City Music Hall's famed Christmas Spectacular show, which draws thousands of people every day to Manhattan's already busy Midtown neighborhood. Stanton had a loaded semi-automatic pistol — which he is licensed to carry — strapped to an ankle holster. The ex-New York cop also had an extra magazine for the gun, containing additional rounds of ammunition, on his belt.

Members of the New York National Guard patrol New York's Penn Station on March 22, 2016. Mark Lennihan | AP

A security guard stopped Stanton and waved a metal-detecting wand around him. The guard "didn't check my ankles," Stanton said, adding that other guards at the venue likely also weren't bothering to bend down to check below patrons' waists. And when the wand detected metal around his own waist, Stanton pulled out the extra magazine but covered it with a set of keys, obscuring it from the guard's sight. "He wasn't even really looking. He heard the jingle of the keys. There was so many people, he was preoccupied .... and he waved me right into the hall," Stanton said. The private eye walked inside to join hundreds of other people, many of them children, waiting for the legendary Radio City Rockettes to take the stage and kick off the show. "I'm one of the good guys," Stanton said. "But if I went in with evil intent, and three other guys, we could have killed many, many people."

The potentially deadly security breach occurred in a city that was the site of the 9/11 terror attacks that destroyed the World Trade Center — and just a few blocks from Times Square, where a terrorist named Faisal Shahzad unsuccessfully tried to detonate a car bomb in 2010. As a result of the 9/11 attacks, $650 billion has been spent on homeland security to, among other things, beef up protection at sites such as airports, train stations, sports stadiums and other places that draw large crowds. But Stanton's story, repeated security failures at America's airports, and other lapses elsewhere underscore the fact that even after all of that spending, the danger of terrorists pulling off a successful attack in the United States, like the attacks seen Tuesday in Brussels, remains a very real possibility. Stanton said that every terror attack is followed in the United States by demands from politicians to improve security. "Then, after a brief period of time, a malaise sets in," and people — including security screeners — become more complacent, Stanton said. "If it doesn't happen immediately, we think it's not going to happen," he said.

Anthony Roman, whose Roman & Associates firm does investigation and risk-management analysis, said the United States has done a better job than Belgium and other European countries at implementing security measures designed to thwart terrorism. "There's a fundamental difference in the Belgian, European and American intelligence penetration of the radical cells," Roman said. "The American penetration is deeper and more efficient on both the human level and on the cyber level." Roman pointed out that New York City's police department has managed "to prevent about 21 attacks in the past couple of years" as a result of its own intelligence collection and intelligence shared by federal authorities. But Roman also noted that "prevention at airports has remained ineffective in Europe and in the United States." "The passenger terminals where you buy your tickets, and the baggage claims areas [have] wholesale inadequate protection," Roman said. He also said that "vehicles are still allowed to loiter for long periods of time at curbside," where they are supposed to be dropping off or picking up passengers. That security hole, coupled with the lack of sufficient investment in bomb-resistant architecture at airports, make airports much more vulnerable to an attack using an fertilizer-fueled car bomb, he said.

Roman also said that security at U.S. airports is "insufficient" around runways and plane taxi paths, because of "an over-reliance on electronic security measures," such as sensors, to detect breaches of the perimeter around those areas. "When those things break, there's insufficient manpower, particularly in the New York metro area ... to mount irregular but constant patrol of the perimeter" to compensate for the loss of electronic eyes and ears, Roman said. He said the patrols should be "irregular" so that their pattern can't be anticipated by terrorists. Why don't airports have adequate manpower to perform those measures? "Money, money, money," said Roman.

Roman said that if more media attention was given to holes in security on airport perimeters, government officials would be willing to spend on filling those gaps, in much the same way they fund air passenger screening. But he also noted that even though the Transportation Security Administration "has sufficient income to operate effectively," that agency, which is responsible for airport passenger screening, has done a "poor" job of planning and executing its mission.

Last June, the acting head of the TSA was reassigned after an investigation by the Department of Homeland Security found that agents posing as passengers had managed to get weapons passed TSA screeners 95 percent of the time. "It's horrible," Roman said of the TSA's performance in those tests. "Here's a case where you have sufficient funds, but you have insufficient execution." In response Tuesday a TSA spokesman pointed to testimony before a congressional committee last November by TSA Administrator Peter Neffenger, who said "I was greatly disturbed by TSA's failure rate on these tests, and have met with the inspector general on several occasions to better understand the nature of the failures and the scope of the corrective actions needed." But Neffenger, in that same testimony, also said, "this was not a deliberate test of the entire system, and while there were areas for improvement noted by the inspector general — with which we concurred — [it is important to acknowledge] that the system as a whole remains effective and, as a result of this series of tests, has only gotten stronger."