It used to be public transport that was the favoured place for terrorists and deranged killers.

Now it seems concerts are becoming their preferred hunting grounds.

The atrocity at the Bataclan concert hall in Paris in November 2015, the bombing at the Ariana Grande concert at the Manchester Arena this year, the Orlando Pulse nightclub attack and now the appalling massacre in Las Vegas has many asking if a deadly new trend is emerging.

Experts in the US believe entertainment events are soft targets for people intent on creating mayhem.

Belongings are scattered and left behind at the site of the mass shooting at the Route 91 Harvest Festival in Las Vegas. Photo / Getty Images
Belongings are scattered and left behind at the site of the mass shooting at the Route 91 Harvest Festival in Las Vegas. Photo / Getty Images

"What [gunman Stephen Paddock] did was he identified a gap in security and he exploited it," former LAPD Officer Steve Gomez told ABC News.

"It now shows the vulnerability that's out there."

John Cohen, a former counter-terrorism co-ordinator for the US Department of Homeland Security, listed concerts at the top of the soft target list, which also included transportation hubs and shopping malls.

This meant attackers can be less trained and prepared than if a more secure location - for example a government building - was targeted.

"You can essentially get your weapon, go to a public place and kill or injure as many people as you can," Cohen said.

Preventing such attacks was not easy.

"It is extraordinarily difficult to secure every soft target within a jurisdiction, so our success in reducing these types of attacks will only come when we're better able to identify those within our communities who are potential attackers and prevent them from committing an act of violence," Cohen said.

Terrorists will be watching

Deakin University counter-terrorism expert Greg Barton told there were specific local dimensions to the Las Vegas shooting.

Shortly after the shooting, Isis claimed responsibility, but so far no evidence supports this.

"So far, despite the [Isis] claim, there doesn't appear to be a terrorist link. Police would be able to tell from his social media consumption. Of course, in Nevada there's open carry laws and you don't need permits to purchase assault rifles."

The Route 91 Harvest country music festival bracelet worn by a survivor on October 2, 2017 in Las Vegas. Photo / Getty
The Route 91 Harvest country music festival bracelet worn by a survivor on October 2, 2017 in Las Vegas. Photo / Getty

The fact it happened in the United States, which has well known problems over gun ownership, made the massacre more complex than more traditional acts of terrorism.

"The audio recording does suggest the gunfire was coming at automatic rates from different guns and they found 23 guns back at his room, explosives, thousands of rounds of ammunition."

Barton said taking guns into rooms like Paddock did was relatively easy. "Bizarrely enough in that hotel, and in most hotels in Las Vegas, you can pull into basement carpark pull out your duffel bag full of weaponry and go into the lifts and no one checks."

It was only when someone walked onto the casino floor they had their bags checked.

"So clearly it was premeditated and carefully thought out by someone who knew what he was doing. The question really is [one of] motive. If it does come to light he's perhaps a psychopath that might explain the callous cold-bloodedness. He doesn't seem to be a guy with a gun fetish - and yet he was very effective with what he did."

Even though a terror link had not been established Barton warned terror groups like Isis would be carefully watching how lethal this attack had been.

"The worrying thing is having demonstrated how effective this is, someone will copy it. Probably this guy isn't motivated by a political issue, so he is not a terrorist, but the message won't be lost on terrorist groups - that this is an effective way [to kill many]."

Using a high-rise hotel to shoot from meant there was no security to bypass, less risk of being interrupted and being elevated gave a lethal line of sight.

"That's now been demonstrated. Just like use of trucks, planes, cars as weapons has been demonstrated and repeated, I can see someone repeating this [attack]."

US Homeland Security Adviser Fran Townsend described how the Mandalay Bay Resort became such a soft target in a CBS interview.

"It's also connected to the convention centre and a shopping mall, which means you've got multiple points of entry and exit. It would be very difficult to control that, which essentially makes it a soft target," she said.

In the future venues like hotels might have to start searching hand luggage, or possibly even requiring guests to walk through metal detectors.

Measures such as these were introduced in Mumbai after the 2008 attacks. But Fox News reported they were less likely to be welcomed in the US.

"We saw a change in Mumbai, but we have a different culture in the United States," law enforcement and security expert Paul Viollis said.

But even with all the protection in the world at venues like concert halls, recent terror activity targeting the areas around them was what was having such deadly impact.

Louis Marciani, the director of the National Centre for Spectator Sports Safety and Security, told the New York Times it was hard to imagine how a similar assault might be prevented. "There's no way that any good operation would have caught that," he said of the shooting. "We've now got to go back to the drawing board."