In the wake of the attack in Berlin, there's growing concern that crowded streets and shopping malls are prime targets for violent extremists during this holiday season. The concerns grow exponentially when you're talking about a big city like New York.
Would you be able to recognize a terrorist on a crowded street. Any one of these Christmas shoppers in the heart of Manhattan could be one - it's a dilemma police face every day.
"It's not possible to spot a terrorist just by the way they look, to do so would be futile," said Robert Mccrie, John Jay College of Criminal Justice.
Back in November U.S. authorities issued a somber warning to Americans to be on their guard in the run-up to Christmas around well-known sites.
A bulletin from the Department of Homeland Security read: "The holiday season, in particular, provides additional opportunities for violent extremists to target public events and places where people congregate."
According to analysts, the main worry is the lone wolf terrorist - acting without formal backing from ISIL or any other extremist group.
"It's the number one concern and there's so little that can be done about it," said Robert Mccrie. As we've seen with the attack on a Christmas market in Berlin this week.
And last July in Nice, France, when a man barreled a truck into a packed crowd near the waterfront, killing more than 80 people before he was gunned down by police. ISIL called him "a soldier of the Islamic State" - but hinted it had no prior knowledge of the plot.
John said, "Looking at this map of the United States I've got to believe the biggest threats are to New York, Washington, LA and San Francisco -- is that true "
Mccrie said, "They are true but there are threats anywhere, anywhere there are people who have a rage against life in this country and feel they have no option than to indiscriminately kill people."
So, what are police doing to head-off attacks. In the case of trucks used as weapons, Professor McCrie says officers have been to almost every rental site and many dealerships in the area for a quiet word with managers.
"The policemen's been very polite, here's what we want you to look out for and they've been given some characteristics and just give us a call, here's our card if you have any concerns whatsoever," said Robert Mccrie. He stresses terror attacks are extremely rare.
Nonetheless - as you can see the police are out in force at this holiday market in Columbus Circle - because the reality of life in 2016 is that even holiday shopping carries risks. All we can do is remain vigilant - and hope if there are any planned attacks- that the authorities can catch them in time.
