Prior to the shooting, police say several unarmed fare enforcement officers got into an altercation with two train passengers when one of the men refused to provide proof of fare.
The passengers, later identified as Oscar Perez Giron and Mario Parra Cetina, were asked to get off the train at the SoDo station, where the argument continued.
At about 4:10 p.m. one of the fare enforcement officers called for backup, and a King County Sheriff's deputy arrived on scene moments later.
After the deputy arrived, police say Giron tried to run away. The deputy and several fare enforcement officers grabbed Giron, which set off a brief scuffle.
During the melee, Giron pulled out a handgun and pointed it at the deputy's head, according to police. The deputy then opened fire on Giron, killing him.
Moments after the shooting, Cetina rushed toward the deputy and grabbed him. Video shows him quickly backing away when the deputy points his gun at the man.
The deputy later searched Cetina and found a stolen handgun in his backpack. The gun wasn't loaded, but Cetina had two magazines of ammunition and another box of ammo in the bag.
He was arrested and booked into King County Jail for investigation of assault and unlawful possession of a firearm.
Seattle homicide detectives are investigating the shooting.