Jason Blossom's Death is an event that took place prior to the series, and it is the overarching theme of Season 1.
Fateful Morning of July 4th
As the original story went, just after dawn, on the 4th of July, Jason Blossom and his twin sister Cheryl drove out to Sweetwater River for an early morning boat ride. However, the trip ended disastrously, with Jason supposedly drowning as he attempted to retrieve Cheryl's glove that had fallen into the water. Cheryl was found by Dilton Doiley and his scouts, soaking wet and crying her eyes out, by the shore. It was not until weeks later that Jason's body was found by Kevin Keller and Moose Mason on the river shore, with a gunshot in his head.
Following Jason's autopsy, the local corner discovered that he was subjected to animal scavenging, ligature marks, and signs of him being frozen. It was also revealed that despite popular belief, Jason died on about July 11th, not July 4th.
Cheryl is Guilty?
Upon Cheryl's arrest, she admitted to lying about the events that transpired on July 4th. Jason wanted to leave Riverdale for good, so Cheryl helped him fake his own death to ensure that nobody would come looking for him. The part of the story that Cheryl told about Jason falling into the river and drowning was not true. Jason never fell into the water, in fact, they both made it to the other side perfectly dry, but that was the last time Cheryl saw him, on the other side of Greendale, where they parted ways. He was supposed to contact her when he made it to a safe place, but he never did. Cheryl had no idea what happened to him after that, or how he ended up dead but she did hear a mysterious gunshot that day.
In an effort to protect his family, Hal Cooper broke into Sheriff Keller's home, and stole all the evidence in the Jason Blossom murder case, ranging from the documented files, background checks, and the video and audio tapes of police interviews.
Romeo & Juliet Escape Plan
According to Polly Cooper, the reason or at least one of the reasons for Jason Blossom wanting to fake his own death was so that he could run away with her. They had planned to do so on July 4th, after learning that Polly was pregnant with his child. They were going to meet on the other side of Sweetwater River but that never happened. Betty Cooper and Jughead Jones discovered Jason's getaway car on a highway off route 40, inside the vehicle, they found Jason's belongings, such as his varsity jacket and copious amounts of drugs. However, by the time the police got there, the vehicle had been set on fire.
Jughead was taken into custody and interrogated down at the Sheriff Station by Sheriff Keller, who learned that Jughead had a long history of being bullied by the football team, which would have likely included Jason. Sheriff Keller suspected Jughead was angry after being picked on for such a long period of time by a privileged rich boy, while he was a poor kid from the wrong side of town with a deadbeat dad. This would give Jughead motive to kill Jason. However, he was later released after Fred Andrews claimed that Jughead was working for him the day Jason was murdered. Notably, the police might have taken the wrong Jones into custody, as FP was in possession of Jason's varsity jacket, which was previously seen in Jason's car before it went up in flames.
FP—A New Suspect?
After Sheriff Keller searched his trailer and found the weapon used to murder Jason, FP Jones was taken into custody and claimed to be guilty. Under interrogation, FP falsely confessed that he had made a deal with Jason to sell drugs in exchange for cash and a get-away vehicle. Unfortunately for Jason, FP eventually learned that he was the son of Cliff Blossom, leaving him to wonder why a rich kid like Jason would need the help of low life gang member. Jason explained that Cheryl was going to row him out to Sweetwater River on July 4th, tip the boat, and claim that he drowned. When the plan commenced on the morning of July 4th, Jason made it across the river where FP claimed he abducted him and took him into the basement of the Whyte Wyrm, where he stayed tied up until July 11. The plan consisted of them getting word to Jason's father that he was alive and well, but that his safe return would be reliant upon a large amount of money being paid forward by the Blossoms. Before he could send word to Cliff, FP caught him trying to escape, and that's when he shot him, froze his body, and dumped it in the river. His one call was to Joaquin. Veronica and Archie, however, had searched the trailer earlier that day and the gun was not there, so they believed it had been planted.
Jason's Killer Revealed
Kevin, Archie, Betty, Jughead, and Veronica went to ask Joaquin about the call from FP. At first, he avoided the details, but then revealed he had said to forget about the contingency plan because it was too dangerous; he said it involved a box but he hadn't a clue what was inside. Joaquin took them to the location where the box was buried and then left. Inside was Jason's letterman jacket. At first, they didn't understand the danger. However, thanks to creative thinking by Betty, she had Archie put on the jacket and when they checked the pockets, they found a hole. Inside the jacket's lining, they discovered a USB drive.
The five plugged in the USB and it revealed security camera footage from the White Wyrm basement on July 11. It showed Mustang taunting and beating Jason, who was tied to a chair, before Cliff Blossom entering the basement. He took Rose Blossom's ring from Jason's pocket, and moments later took out a revolver and shot Jason in the head. FP and Joaquin found the body and froze it until they later dumped it in the river, so that part of the story was true. FP was no longer guilty of murder, but several other things remained. Cliff's motive was, he revealed to Jason, that the maple syrup business was a cover-up for his true livelihood—drug dealing. Jason wanted no part of the family business, prompting his urge to run away. Cliff was furious and feared Jason would lead to his arrest, so he did what he thought he had to do. Cliff was found hanging in the barn (where the Maple Syrup and drugs were stored) when the police came to arrest him. The reason FP covered for him was that on the day Cliff visited him, he warned that if he didn't take the blame, Jughead would suffer the same fate Jason had. 
Images Provided By the Official Tumblr for the CW's Riverdale.