Marcus Yallow is a 17-year-old hacker/techno whiz from San Francisco. One day at his high school named after Cesar Chavez, Marcus is accused of hacking into the school by vice principal Frederick Benson. Marcus and Benson do not get along and Marcus believes that Benson is perpetually trying to get rid of him. Marcus is let go due to a lack of evidence and he returns to class.
Later that day, Marcus and his best friend Darryl escape school to play a massive augmented reality role game that partially involves a scavenger hunt around town. They meet up with fellow group members and game players Vanessa and Jolu. While searching for a part for the game, a series of explosions go off in the city. This sets off sirens and alarms which sends everybody running for the shelters. The group initially tries to go to the shelter but then they decide to leave. While leaving, Darryl is stabbed in the crowd. They finally escape the crowd and flag down a vehicle because Darryl needs immediate medical attention. Three men get out of the vehicle and put bags over each group members' head and then shove them into the vehicle.
Marcus and his friends arrive at an unknown location and are put into a trailer where they are separated and are amongst many other people. They find out that they are being held by the Department of Homeland Security (DHS) to be interrogated for suspicions of being connected to the terrorist attack. After a series of interrogations that take place over a period of six days, Marcus, Jolu, and Vanessa are finally released. Darryl's whereabouts are unknown. The DHS tells Marcus that they will be monitoring his actions and moves because he is still a suspect. Marcus is "infuriated at how his civil rights [are] ignored."
Marcus revolts by setting up technological attacks on the DHS in order "to [thwart] further efforts to restrict personal liberty". Marcus also sets up a network primarily composed of teenage members using a game console so they can communicate freely while fighting "the surveillance state." This causes a war between the rebelling youth and the Department of Homeland Security.
When a former prisoner who was held by the DHS tells Marcus that Darryl is still alive, Marcus tells a reporter and his family about his actions taken against the DHS. The report from the reporter is then published and the DHS takes Marcus into custody again. During a waterboarding interrogation, highway patrol troopers raid the DHS compound because of an order from the governor and arrest the DHS agents. Darryl is subsequently freed and Marcus returns to his life the way it was before the terrorist attacks.

Question: What set off the sirens and alarms in the city?
Answer: The explosions
Alternative answer: Explosions set off the alarms in the city.