Eight people died and many others were injured at the Astroworld Festival in Houston, the latest in a long line of tragedies in which people were crushed to death at major events.
Crowds surged forward as rapper Travis Scott took to the stage at the outdoor music festival on Saturday, during the first day of an event that attracted tens of thousands of people.
Disasters such as this are common at events that attract large crowds.
In 1988, 93 football fans in Nepal were killed while surging towards locked stadium exits.
And in 1979, 11 people died in a scramble to enter a concert by band 'The Who' in the US city of Cincinnati, Ohio.
While most major events take place without incident, experts said common traits can be seen when tragedies arise. Here is a look at how they happen.
How do people die at big events?
At events where crowd surges occur, deaths are not usually caused by people getting trampled. Actually, people are often squeezed so hard that they cannot get enough oxygen.
When a crowd surges, the force can be strong enough to bend steel. It can also hit people from two directions: from the rear, as the crowd pushes forward, or from the front as people try to escape. If some people have fallen, causing a pile-up, pressure can even come from above. Caught in the middle are people’s lungs.
What causes disasters such as this?
G Keith Still, a visiting professor of crowd science at the University of Suffolk in England, has testified as an expert witness in court cases involving crowds.
“My research covers over 100 years of disasters, and invariably they all come down to very similar characteristics,” said Prof Still.
He said the first issue is the design of the event, including making sure that the density of the crowd does not exceed security and safety guidelines, including having enough space for everyone and large enough gaps for people to move about.
Some venues will take precautions when they know a particularly high-energy crowd is coming to an event. Prof Still pointed to how some will set up pens around stages in order to break large crowds into smaller groups. That can also allow for pathways for security officers or for emergency exits.
What can cause stampedes?
Crowd density may be the most important factor in a deadly surge, but it usually needs a catalyst to get everyone rushing in the same direction.
A sudden downpour of rain or hail could send everyone running for cover, as was the case in Nepal in 1988. Or, in an example that Prof Still said is much more common in the US than other countries, someone yelling “He has a gun!”.
Surges do not always happen because people are running away from something. Sometimes they are caused by a crowd moving towards something, such as a performer on the stage, before they hit a barrier.
Prof Still also cited poor crowd-management systems, where event organisers don’t have strong procedures in place to report red flags or warnings, among the reasons deadly surges happen.
How has the pandemic affected the situation?
Steve Allen of UK consultancy Crowd Safety said it is always important to monitor the crowd, but especially so now that events are ramping up in size after pandemic lockdowns start to be lifted.
“As soon as you add people into the mix, there will always be a risk,” he said of crowds.
He recommends that events have trained crowd spotters with noise-cancelling headsets who are in direct communication with someone in close proximity to the performer who’s willing to temporarily stop the event if there’s a life-threatening situation.
That could be a crowd surge, structural collapse, fire or something else. Allen said he has personally stopped about 25 performances by the likes of Oasis, the Red Hot Chili Peppers and Eminem.