South Korea has declared a period of national mourning for the victims of a crowd surge during Halloween celebrations in the capital Seoul. President Yoon Suk-yeol visited the narrow alleyway in the Itaewon district where people were trapped and crushed, and at least 151 were killed. Authorities are warning the death toll may rise further, with more victims in critical condition. An investigation is underway into what caused the tragedy. Warning: This report includes distressing images.