Attacks like last weekend's bombing in Manhattan can't easily be stopped, according to security expert Will Geddes.
However Geddes, CEO of security firm ICP, suggested that "even the stupidest terrorist" would have picked a different location to let off an explosive device.
Last Saturday 29 people were injured when a bomb went off at the junction of 23rd Street and Sixth Avenue in New York City's Chelsea neighborhood. Another device, a pressure cooker bomb filled with shrapnel, was found a couple of streets away.
Five more suspected explosive devices were also found near Elizabeth train station in New Jersey, one of which exploded when a bomb squad robot attempted to disarm it.
Geddes told Julia Hartley-Brewer: "The location wasn't the best place for even the stupidest terrorist for a max casualty rating."
However, he added: "It's very difficult to stop these type of things, particularly when they are on a low level, guerilla warfare style.
"As much as the New York authorities will have countermeasures to protect certain locations, buildings, and the like, you have such a large footfall of individuals coming into Manhattan.
"It's very, very difficult to spot, but they were relatively unsuccessful."