Chinese President Xi Jinping ordered an investigation Thursday into a stampede that left at least 36 people dead in Shanghai during a New Year’s Eve celebration as families desperately searched hospitals for survivors of the disaster.

Witnesses described the New Year’s Eve crowd on the Bund — Shanghai’s landmark waterfront area — as being packed so tightly that people couldn’t move their arms and legs. Some said individuals were lifted off the ground and tossed about as waves of motion rippled through the throng.

The stampede occurred at 11:35 p.m. Wednesday, according to authorities. The exact cause is unknown, and conflicting reports have emerged in its aftermath. At least 47 people were injured in the chaos.

In an early account Thursday morning, state-run news agency Xinhua quoted a witness as saying the stampede began when people rushed to pick up what looked like U.S. currency tossed from a store on the third floor of a building on the Bund. The paper rectangles were actually coupons for a club.

But witnesses at the New Year’s Eve celebration said the crowd had swollen to unsafe proportions even before the coupons dropped.

A post on the social-media account of the Shanghai police also dismissed the suggestion that the coupons prompted the stampede, suggesting it occurred before the paper notes were tossed to the crowd.

Some people said the panic may have been caused by revelers pushing to climb on and off a viewing platform over the Huangpu River.

“They just kept letting more and more people onto the Bund,” said Scott Moskowitz, 31, a U.S. graduate student visiting Shanghai, who was trapped for more than an hour in the crowd but did not witness the deaths.

“At first, people were just bumping into each other, no big deal,” he said, “but it got tighter and tighter to where you couldn’t even move left or right.”

Xi requested an immediate investigation and said that “a profound lesson should be learned from the incident,” according to government-run media.

The New Year’s celebration was supposed to be scaled down this year because of authorities’ concerns over how large it had grown in the past, with nearly 300,000 people attending last year.

Many Chinese citizens and some domestic media blamed the tragedy on an inadequate police presence and poor coordination by authorities.

Pictures circulating widely on Chinese social media showed people lying on the ground near the waterfront after the disaster and others trying to revive them.

At hospitals in Shanghai, relatives looking for missing family members were shown sobbing and angrily demanding answers from authorities.

Moskowitz said at times the dense crowd moved in waves, forcing his body and those of other people to bend to 45 degree angles and lifting some individuals off the ground.

He and his friends eventually jumped over a wall to escape the surge.

One witness wrote on the Chinese microblogging platform Weibo that people in the crowd fainted, one after another. Some tried to pull people out from under one another, the writer said.

Another witness wrote that the scene afterward was surreal — the ground littered with shoes, backpacks and cellphones and covered with pools of blood and vomit. The faces of some victims were unrecognizable, the witness said online.

Most of the dead were students. The youngest was 16 and the oldest was 36, Xinhua reported. The Shanghai government reported that one Taiwanese citizen was among the victims.

Gu Jinglu in Beijing contributed to this report.