"Corey Johnson has confessed his actions to our investigators stating that he stabbed the victims because of his religious beliefs," said Palm Beach Gardens Police Chief Clint Shannon at the time. "Our understanding is he had converted to Islam and had been watching violent videos online."
Sierra died of his injuries, including a slash at his throat, and Johnson is to be tried as an adult for the crime.
The family filed a lawsuit on Monday against Publix for selling the knife to Johnson just hours before he killed Sierra.
It is unlawful to sell weapons to anyone under the age of 18 in Florida.
"Publix has refused to change their reckless policy of selling knives to underage buyers and has spurned Florida law by intentionally failing to check identification before selling dangerous weapons to underage knife purchasers," said the family's lawyer to WPEC.
"Publix is fully responsible for the damages caused by their illegal knife sale and could be responsible for punitive damages if their conduct rises to the level of intentional disregard of Florida law," he added.
When prompted for a response to the lawsuit, a spokesperson for Publix said it would "inappropriate" to comment on pending litigation.