South Korean authorities are investigating the cause of a crowd surge that killed at least 154 people, including 26 foreigners, during Halloween festivities Saturday night in Seoul, according to the Associated Press. Mourning family members have since gathered to identify their loved ones, and thousands of others are paying their respects throughout the city.

“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,” South Korean Prime Minister Han Duck-soo said, according to the Washington Post.

Seoul’s Itaewon area is a famous location for Halloween parties and has recently grown in popularity among young South Koreans. Local officials estimate that as many as 100,000 people attended celebrations there over the weekend. Saturday’s crowd was the largest such gathering since before the pandemic. However, witnesses recall minimal crowd control leading up to the surge.

Police said that even after the first emergency response teams showed up at around 10:24 p.m. on Saturday, officers had difficulty weaving through the crowd to reach those in distress. Many people reportedly believed the police were fellow partygoers dressed in costume for Halloween.

Investigators launched a 475-member task force to investigate the catastrophe, gathering videos from approximately 50 local security cameras and scouring social media for additional footage. Police are also interviewing dozens of witnesses and survivors, and have called on forensic experts for help determining exactly what led to the tragedy.

South Korean President Yoon Suk Yeol has declared a one-week national mourning period.