In GTA 5 there are many collectibles, among them the Letter Scraps. These letter scraps are associated with the infamous GTA world murder of Leonora Johnson. She was murdered in 1975 by an unknown assailant (that is, unknown until you find all the letter scraps). Her body was mutilated and was discovered on the side of the Land Act Dam with the help of an anonymous tip-off to the LSPD.
The letter was written by Leonora's Killer and is addressed to one "David Richards" confessing his crime. However, the letter was torn apart and 50 pieces in total are now scattered throughout Los Santos. Once you find all the letter scraps you can put them together to read the confession letter. This letter helps you reveal the killer's location which becomes available as a 'Strangers and Freak' mission called 'A Starlet in Vinewood' for Franklin.
The letter scraps can be collected when playing as any one of the main protagonists, however, the main killer confrontation is only available when you're playing as Franklin. When collecting with Franklin, you can also use Chop to help locate the letter pieces. You will be able to tell whether or not you're near the letter piece by listening to paper rustling sound effects.
You can also visit www.whokilledleonorajohnson.com using the in-game browser to read about the murder in detail. The site also helps you grasp some important and critical information.

This guide will help you find all 50 letter scraps which you can use to track down the killer.