Guns

The shooter, Ryan Christopher Palmeter, lived with his parents in a suburb of Jacksonville, Sheriff T.K. Waters told a press conference.

"There was no criminal record, nothing," he said, adding that the only thing on file was a domestic violence call with his brother.

"There were no red flags."

Reuters could not immediately reach members of the shooter's family for comment.

The social network has apologised, saying a glitch had prevented it from properly screening posts and removing those that violated its policies.

Marketplace offers users a way to buy and sell items to and from others in their local area.

An earlier version of the facility used to be available on Facebook's website.