About halfway through Wednesday's One Chicago crossover, the series hinted at the source of the fire that left many injured, and at least one major character dead, being a crime.
After Jay Halstead (Jesse Lee Soffer) said goodbye to his father — who ended up brain dead after injuries sustained in the fire — he jumped into the opportunity to help the Intelligence Unit investigate what caused the incident.
The investigation is kickstarted after a Jane Doe from the residential building runs away from the emergency room, cutting a GPS chip from her body just before escaping.
Burgess (Marina Squerciati) and Jay team up to search a park for the missing girl, and they find her as she is about to pass out from her injuries. The girl tells Jay that she got hurt before the fire started by a man, and he started it to cover his tracks about a murder he had committed. She dies later as the doctors try to save her life again.
As the unit and the firefighters try to figure out what happened in the apartment, Antonio raises his suspicions that the woman might have been a victim of sex trafficking, and the GPS chip might help them track down the other two people that were found dead where the fire first started.
Later in the episode, the team identifies one of the victims as Edgar Torres, a big name in the Sinaloa drug cartel. The new information leads the team to a driver for the cartel named Carlos, who was seen sitting in a parked car next to the building as the fire raged on.
The cops detain Carlos, who is revealed to have two sons, Juan and Daniel — the man who was seen at Med just before Jane Doe ran away — and they attempt to get him to say what happened.
As he refuses, the efforts turn to finding Daniel, who is off the grid and the safehouse they think might be where he is hiding is empty. Frustrated, Jay comes up with the idea to detain Juan, Carlos' other son who has no gang ties, and use him to get Carlos to turn on Daniel.
The plan, while unorthodox, works and Carlos admits he has no power in the cartel, but that Daniel knew that if he took out Edgar hw would be given more responsibilities within the gang. He hesitates to help the cops find him, however, knowing that the second the cartel finds out of Daniel's actions, both he and Carlos will be dead. Jay threatens to send Juan to jail, and have him take the heat for the crime instead, which pushes Carlos to turn on his son.
In the end, Jay disobeys Voight's (Jason Beghe) orders and tracks down Daniel himself. When he tries to catch up to him, Daniel shoots Jay, but before he falls on the ground he also shoots and injures Daniel. The man dies and Upton (Tracey Spiridakos) finds Jay and sees that the bullet was stopped by a bulletproof vest and he will be fine.
His proximity to the case will likely have long lasting effects for Jay moving forward.