The day before Thomma shot his commander Colonel Anantharot Krasae at the Surathamphithak army camp. He then shot and killed the commander's mother-in-law. Thomma then stole a Humvee and drove to the Terminal 21 shopping mall where he left the vehicle and began shooting indiscriminately at people before detonating a cooking gas cylinder, killing 26 civilians. He then took sixteen hostages inside the mall on the fourth floor. Thomma live-streamed on Facebook Live during the siege and shared photos and memes on his profile page. Police officers and soldiers stormed the mall and demanded Thomma's surrender, to which he responded by opening fire on them, killing three policemen and a soldier and wounded at least three others.