A King County sheriff's deputy responded to the scene after unarmed fare enforcement officers struggled with the group on the Sodo platform amid a fare dispute.
One man pulled a gun and wrestled with the transit deputy, the probable cause report says. The deputy drew his weapon while holding the man by his wrist and pointed his weapon at him, reports say. Meanwhile, one of the man's associates grabbed the deputy's arms from behind.
Shots were fired from the deputy's weapon and the deputy freed himself from the grip of the man who tried to hold him, according to court records.
The man who drew the gun on the deputy, later identified as Oscar E. Perez-Giron, died of his injuries on the platform.
Officers took the two men connected to the deceased in for interviews.
Investigators recovered two handguns from the scene - one belonging to the man who brandished his gun and one, a loaded semi-automatic handgun, was connected to the man who grabbed the deputy, according to the affidavit.
The man, 23, denied being involved in the confrontation. Police booked him into King County Jail for investigation of illegal firearm possession and assault.
The third man in the group was released after questioning.