More than 50 people were killed Tuesday and hundreds injured in a crush of bodies during a funeral procession for Maj. Gen. Qasem Soleimani in his hometown of Kerman, Iran, prompting officials to postpone the planned burial of Iran’s notorious military strategist, who was killed by a U.S. drone strike last week in Baghdad.

Unverified videos shared on social media showed mourners huddled around sprawled bodies on the street. They had come to mourn a national figure but found themselves fighting for their lives or trying to resuscitate their compatriots.

It remains unclear what triggered the stampede-like event in southeastern Iran.

Hundreds of thousands of mourners have poured into the streets for funeral processions for Soleimani held in several cities over the past three days, during which crowds have carried the slain Quds Force commander’s coffin above their heads. While Iran’s authoritarian regime at times corrals people to join protests, inflating their numbers, analysts said the turnout spoke to a genuine sadness and anger among Iranians at the U.S. killing of a powerful official.

This was not the first time a massive public gathering ended in a crowd crush. Other mass funerals, along with the Hajj pilgrimage, sporting events, festivals and rallies, have seen similar tragedies. But experts and public officials say they are entirely preventable.

G. Keith Still, a crowd safety and risk analysis specialist, said Tuesday’s incident appeared to follow an all-too-common pattern.

“It’s kind of like squeezing toothpaste,” he said. “You get a very high-pressure situation as people are moving through confined spaces. As soon as the density exceeds the physical boundaries … you can develop crowd crushing.”

Still said conditions can start to become dangerous when a crowd grows to more than six or seven people per square meter. A “shock wave” — as people push from behind while those ahead of them are unable to continue at the same pace — can create a deadly pressure cooker.

Rather than being trampled, people in these conditions usually die from constricted asphyxia.

“Imagine that people are so tightly packed together that they can’t breathe,” Still said. “The pressure is so great on people so they can’t expand their lungs."

Still, who teaches and advises organizers of major world events on crowd management, said he doesn’t use the word “stampede” because the term conjures a false image.

Stampedes, he said, occur among animals, which creates the sense that “it’s not within our realm to control,” Still said. “But, in fact, it is.”

Deadly stampedes from funerals to soccer matches

Despite significant research on the science and psychology of crowds, fatal gatherings continue to occur.

Former Iranian supreme leader Ruhollah Khomeini’s 1989 funeral procession turned deadly when at least eight people were killed in the pressure-filled throng of mourners who filled the streets in multiday processions under a scorching June sun.

In the Iraqi city of Karbala last September, at least 36 people were killed and scores more injured after a rush among Shiite Muslim pilgrims gathered in the holy city for the annual Ashura commemoration.

In the Karbala case, the stampede-like incident began toward the end of the procession as people were preparing to enter into a holy shrine. That is when a few pilgrims fainted in the 100-plus-degree heat — a tragic twist the crowd behind them could not see. The waves of pilgrims kept flowing; people slipped and stumbled into what became a deadly crush.

A rush of people on a bridge crossing the Tigris River in Baghdad in 2005 killed more than 960. Rumors about a suicide bomb attack led panicked pilgrims to push toward a shrine.

Another funeral procession turned deadly in Mumbai in 2014, when at least 18 people died as mourners packed the home of the head of the Dawoodi Bohra Muslim community, Syedna Mohammed Burhanuddin. Authorities attributed the deaths to the police’s poor management of the crowd, coupled with unexpectedly high numbers in attendance.

Three years later, however, a deadly crush prompted debate about the country’s outdated and overwhelmed urban infrastructure after 22 people died trying to avoid a rainstorm at a train station.

“Eyewitnesses described people being trampled under a panicked crowd on a pedestrian bridge at Elphinstone Road station, in a recently developed, upscale office area in south Mumbai,” The Washington Post reported at the time. “Although stampedes are common in India, the incident has sparked an uproar about the risks facing Mumbai’s commuters.”

Fears over failing infrastructure also led to a rush of revelers trying to flee a swaying suspension bridge in Phnom Penh, Cambodia, in 2010. That same year in western Germany, 21 people suffocated to death as they tried to squeeze through a tunnel that was the only entrance to the grounds of the country’s Love Parade music festival.

One of the worst tragedies in soccer history occurred in 1989 in what is known as the Hillsborough Disaster: 96 soccer fans died when police tried to relieve a bottleneck of fans by funneling them into pen areas not meant to hold such large numbers. Only minutes after the game kicked off, barriers gave way, sending people in the pens falling on top of each other or fatally caught in the crowd. Investigations ultimately blamed the police’s poor management of the crowd and the slow response of emergency workers.

“The dead, the dying and the desperate became interwoven in the sump at the front of the pens, especially by the gates,” a government investigation that year of the incident found. “Those with strength left clambered over others submerged in the human heap and tried to climb out over the fence … The victims were blue … incontinent; their mouths open, vomiting; their eyes staring. A pile of dead bodies lay and grew outside gate 3.”

In other cases, fatalities have taken on geopolitical importance. In 2015, at least 769 people died in a crush of Muslim worshipers visiting the holy city of Mecca in Saudi Arabia as part of the annual Hajj pilgrimage. At least 136 of the dead were Iranian, prompting Iran to call for regional rival Saudi Arabia to take responsibility and apologize. Riyadh blamed the victims, who had suddenly surged into two pathways rather than entering at their groups’ allotted time slots, authorities said.

“Stampedes have become less frequent in recent years as Saudi authorities have undertaken major construction work to ease the flow of pilgrims,” The Post reported at the time. “But Thursday’s incident is likely to pressure ­authorities to implement further security and crowd-control measures and intensify calls for ­restrictions on the numbers ­allowed to visit.”

Iranian officials were vociferous in blaming poor planning for the deaths. “Saudi rulers were at fault,” Ayatollah Ali Khamenei wrote in 2016.