The NAACP joined Nolan Wells' family in their legal fight for answers ... and the organization is saying enough is enough as questions swirl about the teen's mysterious death.

The NAACP said Monday it has offered additional investigative resources to Nolan's family ... demanding "truth and transparency" in the case that's captivated the nation.

As you know, Nolan went missing after taking a boat out to Horn Island with friends on the Fourth of July. His body was found days later in the water, near where he was last seen alive.

The NAACP's legal arm will work alongside the family's legal team -- which includes civil rights attorney Ben Crump -- as they continue conducting an independent investigation.

Crump already ordered an independent autopsy ... and the preliminary results did not conclude a cause or manner of death.

Crump has previously said that Nolan's throat was unavailable to the Medical Examiner who performed the independent autopsy -- but today, NAACP president Derrick Johnson confirmed to CNN's Jake Tapper that the M.E. will receive the throat before the case goes before a grand jury.

Nolan's friends, who were with him for a portion of that fateful day, have denied any wrongdoing in his disappearance and death.

And while the family has not accused the friends of foul play -- they've made it crystal clear the initial rush to judgment by authorities that it was all a tragic accident was the result of poor police work.