After at least 1,300 people died during this year's hajj pilgrimage in Saudi Arabia, witnesses and experts say a combination of scorching temperatures and crowd control problems made the holy event disorienting and dangerous.
At Muslim holy sites in Mecca, temperatures rose as high as 120 degrees Fahrenheit during the annual five-day pilgrimage. Witnesses described people vomiting and fainting in the extreme conditions as they attempted to perform hajj rituals in the city.
"People fell on the streets," said Muhammed Adnan Chishti, a 37-year-old Islam teacher from Hong Kong.
He said street closures near Mina, where pilgrims perform a ritual symbolizing the stoning of the devil, meant people had to walk for hours in "scorching heat".
"What was supposed to be a 10-minute walk took three to four hours," Chishti said, adding that many pilgrims were unaware of the route closures, which authorities had introduced to control crowds. The flow of people along the alternative detour left many baking in the sun for hours, he said.
"I saw a woman standing next to me," Chishti said. "She fainted due to the intense heat. We started pouring water on her to cool her down and gave her vitamins, but she fainted. Then she vomited on my leg. The situation was terrible."
Chishti, who traveled alone, said he himself felt the effects of a heat-related illness on June 11.
"I was so dizzy I didn't know what was happening to me," he said, adding that he still feels some lingering symptoms, including weakness and dizziness.
According to The Associated Press, more than 1.8 million Muslims have performed the pilgrimage this year.
Deaths during the Hajj have been reported in recent years, but not always due to extreme heat. In 2015, more than 2,400 people were killed in a stampede in Mina, the AP reported. In other years, pilgrims died in accidents or disease outbreaks, according to the AP.
However, this month's unusually high death toll has sparked increasing scrutiny over whether authorities took the right precautions as crowds gathered amid rising temperatures.
In a statement, Saudi authorities said they had responded quickly and provided "effective support to the Hajj security forces in managing and mitigating the effects of heat stress."
The Health Ministry said "significant efforts" had been made before the Hajj to "raise awareness about the dangers of heat stress and the importance of preventive measures."
Saudi authorities also said healthcare providers had treated people for heat stress and provided emergency care, including dialysis, cardiac catheterization and open-heart surgery. More than 30,000 ambulance and 95 air ambulance services were provided, the Ministry of Health said.
Saudi Health Minister Fahd bin Abdurrahman Al-Jaljel said 83% of the deaths were unauthorized pilgrims who had walked long distances in the heat. Official hajj permits typically include dormitory or hotel accommodation and transportation around Mecca, but many people cannot afford these, which can cost up to $10,000. As a result, dozens of unauthorized pilgrims often set out without proper visas or registration.
"The health system provided more than 465,000 specialized treatment services free of charge, including 141,000 services to those who had not received official permission to perform Hajj," the Health Ministry said.
Chishti agreed that unauthorized pilgrims were disproportionately affected by the heat because they did not have official travel arrangements or proper accommodation.
"All hotels and dormitories had air conditioners and fans. But those who did not have reservations or could not find their assigned hotels were forced to sleep on the streets," he said. "They had no umbrellas or water bottles. There were people who arrived themselves without any booking; they were the most affected."
Shakoor Hajat, professor of global environmental health at the London School of Hygiene & Tropical Medicine, said risk mitigation for the hajj is particularly difficult.
"The hajj is a unique situation because pilgrims are often elderly and walk for long periods of time in very extreme temperatures," Hajat said.
Extreme heat can be fatal because it affects the body's ability to regulate its internal temperature. When conditions are too hot and humid, sweat cannot evaporate and the body can no longer cool itself. Heat can also worsen pre-existing health problems, increasing the risk of heart attacks, breathing problems and kidney disease.
Groups at increased risk include young children, pregnant women and the elderly.
The recent deaths highlight the difficulties in maintaining certain cultural practices and mass gatherings as climate change increases the frequency, intensity and duration of heat waves.
The nature and scale of the hajj make it a difficult event to organize even if authorities do their best to prepare, says Milad Haghani, senior lecturer in urban mobility, public safety and disaster risk at the University of New South Wales in Australia .
"If it is an entertainment event, we will take swift action to cancel the event," Haghani said. "But none of these are an option when it comes to Hajj. It has to happen in a specific location and in a specific city, at a very specific time."
As the intensity of the extreme heat increases, Haghani added, Saudi Arabia will have to reconsider how to keep people safe during the holy pilgrimage. Pilgrims will also need to be better informed about the risks.
"The largest mass gatherings of all time are taking place in Mecca," he said, "and more Muslims are coming every year."
Mithil Aggarwal reported from Hong Kong, Denise Chow from New York and Natasha Lebedeva from Washington, D.C.This article was originally published on NBCNews.com