Would a trial still be necessary? If it is, what could the accused can
bring to the court to be declared innocent?
Yes, absolutely. Consider that it's entirely possible the agent simply doesn't have access to all of the facts - and may not even know that some are missing.
Bob and Mary are in the kitchen making dinner. Mary is cutting strips of meat with a rather large and very sharp knife. Bob takes a step towards Mary, she turns toward him, raises her hands and the knife edge lands in Bob's heart. We'll add another wrinkle: they've been arguing.
Based on evidencial review by an Agent: Mary is determined to have killed Bob and therefore goes to jail.
Now, let's get Mary out of jail with a bit of exculpatory evidence:
Earlier in the day Steve performed work on the kitchen floor. The floor is such that the wood part is screwed into the subfloor. Steve failed to fully seat one of the screws. When Bob took a step towards Mary he had slightly stumbled. Mary had raised her hands to prevent Bob from bumping against her. In the immediate confusion, she still held the knife and it plunged into him with no malice on her part.
If the Agent didn't know that Bob had tripped then Mary's guilt would seem open and shut. The Agent might not even be motivated to search for additional data because the information he had was quite clear. Even if the Agent wanted to search for additional data, finding out that a handyman failed to finish putting a screw in might not be recognizable as relevant.
However, Mary would have the motivation to show that she never intended to hurt Bob. She would likely know that Bob had tripped and be able to point to the spot he tripped at. The story of the exposed screw might even save her.
Moral of the Story: Having near perfect data about the actions of the participants might not be the whole story. You have to let the defense have a turn to present another viewpoint.