King County prosecutors contend Mario Parra assaulted Sheriff's Deputy Malcom Elliot during the June 30 altercation that saw Elliott fatally shoot Oscar Perez-Giron.
Prosecutors contend Parra and Perez, both 23, were both carrying pistols. Perez is said to have been holding his when he was fatally shot after Parra grabbed the deputy's gun hand from behind.
The day of the shooting, Perez, Parra and a third man had been stopped by fare enforcement officers at the Sound Transit station. Elliott, assigned to King County Metro Transit Police that day, intervened to verify Perez's identity.
Video of the incident shows Elliott, accompanied by three unarmed fare enforcement officers, picking up Perez's bag. (Fare enforcement officers check tickets to ensure passengers pay for their trips.) Perez then appears to try to break away.
Writing the court, Seattle Police Detective James Cooper said Perez was grappling with a fare enforcement officer while holding a pistol in his right hand.
Elliott held Perez's wrist with his left hand and drew his gun with his right, which Parra then grabbed, Cooper said in charging papers. Perez was then shot.
The deputy broke away from Parra immediately after the shot rang out, and Parra promptly put up his hands while held at gunpoint, Cooper continued.
Searching Parra's backpack following the shooting, officers found a loaded Glock pistol as well as two spare clips, the detective said in charging papers.
While Parra's criminal record wouldn't preclude him from gun ownership - he's been convicted of low-level drug offenses and car theft, all of which he pleaded out as misdemeanors - it is illegal in Washington to carry a concealed weapon in public without a license.
"The defendant illegally possessed the firearm when he had it concealed on his person on the Sound Transit train, on the platform and when he assaulted the officer," Senior Deputy Prosecutor Jennifer Worley said in charging papers.
Questioned by detectives following his arrest, Parra claimed he was playing with his phone and didn't witness the altercation or the shooting that followed, according to charging papers.
On Tuesday, a small crowd gathered at Sodo Station to remember Perez and protest the shooting. Some suggested fare enforcement officers engaged in racial profiling when they stopped Perez to check his ticket.
"Just because of how you dress, you shouldn't be judged," Perez's cousin Michelle Aguilar told The Stranger. "I will not rest until justice is served."
Seattle police are investigating the shooting at the request of King County Sheriff John Urquhart. A review is ongoing.
Parra, who is also identified as Mario Jesus Parra-Cetina in charging papers, has been charged with third-degree assault and carrying a concealed pistol. He remains jailed on $100,000 bail.