Day 1: Friday -sigAI- (1日目: 金曜日 -屍骸-, 1-Nichi-me: Kin'yōbi -shigAI-; lit. Day 1: Friday -Corpse-) is a chapter featured in AI: THE SOMNIUM FILES and the AI: THE SOMNIUM FILES DEMO.
It is the very first chapter of the game. It covers Kaname Date's investigation of Shoko Nadami's body and his discovery of a witness at the crime scene.
Case File Synopsis
A cold and rainy Friday night in November. The body of a woman was discovered at the merry-go-round in the abandoned amusement park known as Bloom Park... The corpse was bound and displayed on one of the horses.
Date and Aiba arrived at the scene and began their investigation. The victim was Shoko Nadami, an acquaintance of Date's. She was missing her left eye, likely taken by the culprit. Then came a sound from the central column. Thinking quickly, Date ran up to the column's door and shot it open to discover a girl inside. Mizuki. She was shaking from the shock and clenching something in her hands: an ice pick with bloodstains on it.
Story Summary
Bloom Park, Friday 10:54 PM

Special Agent Kaname Date and his assistant Aiba arrived at Bloom Park to investigate the situation, per the request of his Boss, the commander of the ABIS team. Despite being special agent assigned to Psyncing, and therefore not frequently investigating crime scenes, Date was called in due to his connection to the victim, an old friend of his. When he and Aiba arrived at the scene, Boss had arrived, and policemen were already securing and investigating the area, while a forensic inspector named Araya Kagami was examining a footprint left on the ground, before the rain could wash it away. Boss briefly filled the pair in on the situation behind the discovery of the crime scene, so Date began investigating at a distance while Inspector Kagami finished his work.

The body of Shoko Nadami had been tied up to one of the horses inside the merry-go-round, the closest to its entrance. She had been stabbed multiple times by a weapon resembling an ice pick, and her eye had been removed prior to her death. Boss noted that the murder appeared to be a copycat crime similar to a case 6 years prior, but when Date pressed for more info, she refused to answer. Suddenly, after Date finished examining the body, the group heard banging noise coming from the central column of the merry-go-round. Date used Aiba's X-ray vision to see someone inside. Date rushed towards the column, despite Kagami's warning about the footprints, and knocked on the locked entrance to the column. When the person did not respond, Date pulled out his Evolver and shot the door's lock open, only to find the 12-year-old Mizuki Okiura inside, holding a bloody ice pick in her hands.
Central Hospital, Friday

After discovering her inside the merry-go-round, Date took Mizuki to Central Hospital in the Minato District. While sitting at her bedside, Date fell asleep, and began to have a strange dream.
Trivia
Etymology
- The name sigAI (屍骸) translates to "Corpse," which is in reference to the corpse of Shoko Nadami found in this chapter.
General
- The chapter sigAI is the only Investigation chapter in the game to take place on Day 1: Friday, and the only chapter explicitly taking place on Day 1, as SINKIN' IN THE BRaiN could occur anywhere between 11:30 PM on Friday and 9:00 AM[1] on Saturday. This is the only Day in which this occurs, as all other Days throughout the game have multiple chapters spread across daytime and nighttime segments, as well as chapters in alternate story routes.
- In the AI: THE SOMNIUM FILES DEMO, the Central Hospital section is not included. Only the Bloom Park segment is present, and the demo immediately jumps to the chapter PSYNCIN' IN THE PaiN.