SEOUL, South Korea (AP) — South Korean police investigated on Monday what caused a crowd surge that killed more than 150 people including 26 foreigners during Halloween festivities in Seoul in the country’s worst disaster in years, as President Yoon Suk Yeol and tens of thousands of others paid respects to the dead at special mourning sites.

Saturday’s disaster was concentrated in a sloped, narrow alley in Seoul’s Itaewon neighbourhood, a popular nightlife district, with witnesses and survivors recalling a “hell-like” chaos with people falling on each other like dominoes. They said the entire Itaewon area was jammed with slow-moving vehicles and partygoers clad in Halloween costumes, making it impossible for rescuers and ambulances to reach the crammed alleys in time.

Police said they’ve launched a 475-member task force to investigate the crush.

Officers have obtained videos taken by about 50 security cameras in the area and are also analysing video clips posted on social media. They have interviewed more than 40 witnesses and survivors so far, senior police officer Nam Gu-Jun told reporters Monday.

Other police officers said they are trying to find exactly when and where the crowd surge started and how it developed. They said a team of police officers and government forensic experts searched the Itaewon area on Monday.

“The government will thoroughly investigate the cause of the incident and do its best to make necessary improvements of systems to prevent a similar accident from recurring,” Prime Minister Han Duck-soo said at the start of a government meeting on the disaster.

The dead included 26 foreign nationals.

Halloween festivities in Itaewon have no official organisers. South Korean police said Monday they don’t have any specific procedures for handling incidents such as crowd surges during an event that has no organisers.

Police said they dispatched 137 officers to maintain order during Halloween festivities on Saturday, much more than the 34-90 officers mobilised in 2017, 2018 and 2019 before the pandemic.

Citing those figures, police dismissed as “different from the truth” speculation that a police station in the area was understaffed because it was providing extra security for Yoon, who earlier moved the presidential office to a site near Itaewon. They said police-provided security for presidents has long been handled by two special police units which have nothing to do with the Yongsan police station, whose jurisdiction includes Itaewon.

Some witnesses said the stampede was caused by people in the upper part of the downhill alley pushing others in the lower area. Local media also questioned an apparent lack of safety preparations for the large crowd expected to gather in Itaewon.

Asked about those possibilities, Nam, the police officer, said the investigation would look into all possible causes of the crush.

President Yoon on Sunday declared a one-week national mourning period and ordered flags at government buildings and public offices to fly at half staff.

The government opened special memorial sites on Monday in Seoul and other major cities. Tens of thousands of people including Yoon and other top officials visited the sites, placed white flowers and bowed deeply. Many people also laid chrysanthemums, bottles of Korean “soju” liquor, candles and snacks near an Itaewon subway station and posted a host of condolence messages.

Following the disaster, many hotels, department stores, amusement parks and other businesses cancelled Halloween-themed events.