Aaron Carter's cause of death -- drowning as a result of being under the influence -- doesn't add up for his fiancée, which is why she wants a second opinion as to how he died.
Melanie Martin -- who shares a child with Aaron -- tells TMZ ... she's not satisfied with the findings of the L.A. County Coroner's Office -- which ruled Aaron's death an accident based on an autopsy following his sudden death last year.
The reason ... she says there are still elements surrounding the circumstances of his passing that don't quite make sense to her -- like the fact he was partially clothed in the bathtub when his body was found ... suggesting to her he was lucid before he died ... which could point to someone else being involved.
Another thing ... there was mention in Aaron's autopsy of a small abrasion on his left eyebrow and another one at the base of his septum. MM tells us that's a red flag to her and a sign of a possible struggle.
Because of these lingering doubts -- which Melanie says she and her son are entitled to have addressed -- she's mulling over the possibility of having Aaron's case examined a second time by a private investigator and medical professional ... only this time, private ones hired by the family.
Melanie also says she wants the additional investigation to give her peace of mind and closure ... as she believes Aaron himself would want no stone unturned.
Here's the reality ... Melanie tells us she simply isn't sure if she'll be able to pull the resources together to do it all. As you might imagine, it's very expensive, and she says she's not equipped at the moment to get it done. Still ... she's considering her options.
Fact is, Melanie is on the same train as Aaron's mother, Jane Carter -- who recently told us the death scene photos that surfaced screamed foul play ... a theory she doesn't think investigators looked into seriously enough.
Melanie says she's even tried doing a little investigating on her own -- going door-to-door for surveillance footage ... but coming up empty. She tells us she wants the homeless housekeeper who was staying with him to get a second look by cops ... not to mention drug dealers she claims had it out for Aaron, and who might've been supplying him.
In the meantime ... it seems L.A. County officials have closed the book on his death case.