When Charlie Tan allegedly tried to buy a 12-gauge shotgun at a Walmart, he was held up because he was a Canadian citizen but returned with a fraternity pledge, and federal prosecutors contend in a matter of hours, had in his hands the firearm used to shoot and kill his father inside the family's Pittsford home in 2015.

Federal court documents filed this week in advance of Tan's trial on weapons charges laid out what happened on February 5, 2015. According to the court filings, Charlie Tan, then a sophomore at Cornell, entered the Walmart in Cortland around noon and attempted to purchase a Remington pump-action shotgun. After truthfully marking down his citizenship as Canadian, Tan's application was flagged, indicating a delay in approving the purchase.

Tan then allegedly drove back to campus, and recruited his fraternity "little," a fellow Pittsford native in Whitney Knickerbocker, then a freshman. Tan allegedly handed cash to Knickerbocker, who around 2:40 p.m., went into the same Walmart. Knickerbocker was also delayed, this time because the address on his driver's license wasn't current. From there, prosecutors allege both took a trip to the Department of Motor Vehicles, where Knickerbocker updated his information.

Around 4 p.m., the documents state the sale of a shotgun and boxes of ammunition went through, with a clerk escorting Knickerbocker out of the store and handing the goods over. Prosecutors say store video surveillance shows Knickerbocker then getting back inside Tan's waiting car, which took off.

One key detail prosecutors allege happened minutes earlier: Knickerbocker allegedly checked a box indicating the shotgun wasn't purchased for someone else.

Hours later, prosecutors said Tan drove from Central New York to the Rochester suburbs, where he then shot his father Jim in the head. Tan was initially charged with the murder, but the case was thrown out after a jury was deadlocked.

Knickerbocker, who was never charged, is scheduled to testify against Tan at his upcoming June trial.