Eighteen-year-old Cyrus Mason went to Kirkland’s Houghton Beach Park in September to trade his assault-style rifle for a 9 mm handgun and $500 cash in a deal brokered by another teen over Snapchat, according to King County prosecutors.

But in criminal charges this week, prosecutors say the trade was a ruse for a robbery that ended with Mason dead from multiple gunshot wounds.

First-degree murder charges were filed Wednesday against Chrisean Kaufman, 18, and Joshua Hawthorne, 17, both of Renton, and Calvin Vao, 17, of Kent. All three were arrested Wednesday; Kaufman was booked into the King County Jail and the two juveniles, charged as adults, are in custody in juvenile detention, according to prosecutors. The three are each being held in lieu of $2 million bail.

“The three acted in concert to rob victim Cyrus Mason of his AM-15 assault rifle, shot him multiple times on the swim dock of a popular public park, and killed him in the process,” King County Senior Deputy Prosecutor Terence Carlstrom wrote in charging papers.

Kaufman is the last person known to be in possession of Mason’s rifle, and in the days following Mason’s Sept. 16 killing, Kaufman solicited multiple people to buy or trade the rifle in exchange for one or two handguns, Carlstrom wrote. Police have not recovered the stolen rifle.

Hawthorne was charged in juvenile court with first-degree robbery in connection with an unrelated, armed carjacking that occurred a month before Mason was killed, Carlstrom wrote. According to prosecutors, Hawthorne was arrested Sept. 20 after his fingerprints were found on the carjacking victim’s discarded cell phone and a judge released him from custody over the state’s objections.

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Less than two hours after Mason was shot, Vao made a video of himself in the backseat of Kaufman’s BMW posing with the AM-15, Carlstrom wrote in the charges. Two days later, Vao asked Kaufman to help him set up another robbery to obtain two handguns, and more than two months after the deadly encounter with Mason, Vao still had the .22 caliber handgun that was used to shoot him, Carlstrom wrote.

Court records do not yet indicate which attorneys are representing the three defendants. They are to be arraigned March 3.

Kirkland police responded to 911 calls of a shooting around 8:20 p.m. Sept. 16 on the dock at Houghton Beach Park in the 5900 block of Lake Washington Boulevard Northeast, charging papers say. Witnesses reported seeing two people run from the scene.

Mason died at Seattle’s Harborview Medical Center, according to the charges. Police determined eight shots had been fired and Mason had been struck five times; police found four shell casings, either on the dock or in Lake Washington, and also fished Mason’s cellphone out of the water, say the charges.

One of two juveniles who accompanied Mason to the park told police he brokered the firearm transaction on Snapchat with someone later identified as Hawthorne, according to the charges. The juvenile witness also said that person blocked him on the app after he took a screenshot of their Snapchat profile the night of the shooting.

Video-surveillance footage from a house near the park showed a dark sedan drop off two suspects five minutes before the shooting and captured the sound of eight gunshots before the two ran back to the vehicle, which took off north on Lakeshore Drive Northeast, the charges say.

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Another witness told police he’d overheard a phone conversation between Hawthorne and one of the co-defendants discussing robbing someone of a gun and anticipating that things were going to get “hot,” according to the charges.

Police determined the sedan captured on the video footage was Kaufman’s 2008 BMW. He was arrested Sept. 21 in connection with Mason’s killing and his vehicle and cellphone were seized, say the charges.

(The case against the three defendants was not referred to prosecutors for a charging decision until Feb. 12 and each was charged with first-degree murder Wednesday, the same day they were arrested, said a spokesperson for Prosecutor Dan Satterberg.)

Found in the car was a black cloth ukulele case Mason had used to carry the rifle to the park, the charges say. Information from Kaufman’s phone led police to identify Vao as a suspect and in a search of Vao’s apartment, police found a .22 caliber handgun and ammunition; ballistics tests later showed the shell casings at the shooting scene and at least one bullet taken from Mason’s body matched the .22, say charging papers.