"The Seattle Fire Investigator determined origin and cause of the fire," Seattle Fire Department public information officer Kyle Moore responded by email. "We determined on scene that it was an intentionally set fire in the stairwell. We turned the case over to the SPD’s Arson Bomb Squad who conducts the criminal investigation into the matter."
Seattle Police Department spokesperson Detective Renée Witt says that there are no new updates to report, but that they are still asking the public to call the Arson-Bomb tip line if they have any information: 206-684-8680.
That's the way these things work. Once the fire investigators determine it to be an arson (and pouring gasoline on the carpet and lighting it on fire was a dead giveaway), the investigation is handed over to the SPD. So if you have any information on an arson that put 750 lives at risk, give SPD a call.