Police say they don't believe the believed suspect had a motive for the chain of crimes, calling his actions "random and senseless". "We're outraged at what this suspect did".
"It appears that this was just a random act that some type of issue surrounding him, maybe mental health, or substance abuse, or something along those lines may have triggered this", he said.
The suspect was detained by Seattle police after the shootings near Northeast 120th Street and Sand Point Way Northeast around 4 p.m., according to police.
The bus driver, identified by the Seattle Times newspaper and other media as Eric Stark, 53, was shot but managed to turn the bus around and head away from the assailant, police said.
The Times reported late on Wednesday that the gunman was a 33-year-old Seattle resident.
He "acted heroically in the face of extreme adversity to protect his passengers", Constantine said.
Meanwhile, the gunman continued his assault, approaching a second motorist and opening fire, killing a 50-year-old man, before fleeing in the victim's vehicle just as police arrived.
Reportedly, a man opened fire at several cars on the Sand Point Way before crashing his stolen vehicle. "Police were able to take him into custody after a brief standoff", added the Seattle police. He then collided with another vehicle nearby, killing the 70-year-old male driver. The names of the two men killed were not released.
Witness John Barrett told the KOMO television station that he was in his garage when he heard what sounded like fire crackers.
At that point, Garth Green said, police were able to apprehend the man.
The Seattle Division of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives is assisting Seattle police in their investigation, as is the King County Sheriff's Office and Metro Transit Police. Instead, the suspect drove away and crashed into yet another vehicle, killing its driver.
"This is a tragedy for the Lake City community and all of Seattle", said Seattle Mayor Jenny Durkan in a written statement.
The driver has been with Metro since 2012.