(Capcom's primary storyline)
The 1st Investigation Unit was a special team set up in 1998 as part of the Training School Reclamation Project, the refurbishment of the Umbrella Executive Training School for re-opening. This team consisted of Umbrella Security Service personnel alongside an engineering team assessing the superstructure. This unit was further supported by a non-military support team, the 2nd Investigation Unit, which consisted of researchers and administrators.
Following the outbreak at the Arklay Laboratory, Umbrella became interested in re-opening the nearby training school, perhaps as a replacement facility. Project leaders were Drs. William Birkin and Albert Wesker. 1st Investigation Unit's job was to dispose of any bio-weapons and irregular mutants living in the facility, and possibly to destroy paperwork as well, based on evidence in the conference room. To limit the spread of information, a top-secret briefing took place before the mission began where the unit was given as much information as possible.
On 23 July, the 1st Investigation Unit made their way into the facility and began their activities. Early reports include an assessment on the quality of medicines left over. As the investigation progressed to the laboratory complex, they became aware of a cover-up by Umbrella to prevent the publishing of research conducted by Dr. James Marcus, who was conducting experiments on fellow employees in the years leading up to his assassination in 1988. The team was attacked by mutants they were not briefed on, and took severe casualties with members either being mutated into Zombies or eaten by those that did. Though the team in the training school were lost, a small group in the laboratory made their way via cable car to a water treatment plant, which they found had been infested by the leeches that very night. Those in the team not killed in the attacks mutated into Zombies and were subsequently killed either by Rebecca Chambers and Billy Coen, or when the facilities exploded.
- The arrival and subsequent devastation of the investigation unit appear to have taken place on the same day as the events of the game. The Zombies are still feeding on fresh bodies; fireplaces lit for the destruction of research documents are still live, as is the boiler. The Treatment Plant Manager's Diary indicates the facility was ordered to be abandoned immediately before the player's arrival there, with Investigator's Report 2 also being found there. This would put the leech attacks mere hours before the player's arrival, similar to the train stage.
- Resident Evil 0 (2002), file: "Investigation Orders".
- Resident Evil: The Umbrella Chronicles (2007), file: "Reclamation Project Notes".
- Resident Evil 0 (2002), Investigator's Report 2".
- Resident Evil 0 (2002), file: "First Investigation Unit Notes".
- Resident Evil 0 (2002), file: "Investigator's Report".
- Resident Evil 0 (2002), file: "Treatment Plant Manager's Diary".