Well, it could be a rival of Ciel's in business or in a social setting. Maybe someone who is jealous of his status or wealth and sees shooting him as a way to get ahead.
There's also a possibility that it's a hired assassin sent by an unknown party. This party might be aiming to disrupt the current power balance or gain something from Ciel's absence. For example, if Ciel was involved in some political or underworld affairs, his removal could benefit certain groups.
It could be an ex - boyfriend who was jealous and wanted revenge. Maybe he couldn't handle the break - up and decided to harm her in this extreme way.
A rival law enforcement agency member could be a suspect. Maybe they see Deeks as a threat to their own investigations or operations and decide to take him out. It could be a power play within the world of law enforcement.
It could be an old enemy from Beckett's past. Maybe someone they put in jail a long time ago, who has now been released and is seeking revenge.
It could be a random act of violence by a criminal who was in the wrong place at the wrong time. There are some bad elements in the area where Blaine was and they just targeted him without any specific reason other than opportunity.
A possible suspect could be an informant who has turned on Tony. Maybe the informant was promised a better deal by another party if they got rid of Tony. Also, it could be a new criminal on the scene who sees Tony as a threat. This new criminal might be trying to establish their power and reputation, and taking out a well - known NCIS agent like Tony would be a big statement. Another suspect could be a person with a grudge against the NCIS in general, and Tony just happens to be an easy target.
Perhaps it's an insider within the law enforcement or criminal justice system who has a vendetta against the BAU. Maybe they disagreed with the way Derek and his team operate and decided to take matters into their own hands. It could be someone who feels that the BAU has too much power and is trying to send a message.
It could be an old enemy of Ezra. Maybe someone from his past who has a grudge against him for some reason, like a business deal gone wrong or a personal feud.
Perhaps it was someone within Toby's own group who had a hidden agenda. A so - called friend who was jealous of Toby or had some ulterior motives that led them to arrange for the shooting. This would add a lot of drama and betrayal to the fanfiction story.
It could be someone from a case he was working on. Maybe a criminal he put away in the past who has just been released and is seeking revenge. They might have hired some thugs to carry out the beating to send a message to Jamie and the police force in general.
Unfortunately, I don't have information on the author of this particular fanfic. It could be an amateur writer who just wanted to explore this scenario with the character Ciel. There are numerous fanfic communities where such works are published, but without more details, it's impossible to say who wrote it.