Family spokesperson, Jarel (Jarez) Posey tells TMZ ... Coolio's loved ones just got the news from the coroner -- fentanyl killed the rapper -- and he also had traces of heroin and methamphetamines in his system.

Jarez tells us investigators determined Coolio's severe asthma and decades-long use of cigarettes also played a factor in his death and his body's inability to fight back.

Despite the grim news, Jarez tells us Coolio's kids -- Brandi, Jackie, Melan, Christopher, Artis III, Darius and Artisha -- will remember their dad for the great man he was ... and the public should know how much they loved him, and how he conquered the world both on the stage and when he wasn't performing.

We're told Coolio's family has plans to honor their dad through documentaries and film ... and continue the legacy that was his music.

TMZ broke the story ... Coolio was found dead at his friend's house in Los Angeles in September. At the time, Jarez, told us paramedics suspected he suffered a cardiac arrest.

Law enforcement sources told us no drugs or drug paraphernalia were found at the scene.

The world was shaken by the news of Coolio's death -- including his close pal Vanilla Ice, who last performed with the legend just days before his passing.

FYI, Ice got pretty emotional recalling the last time he had a heart-to-heart with the fellow artist ... adding that he even pushed pause on his tour following the rapper's death.

Coolio's family made a headstone so they could have a physical place to go and remember him ... enlisting Cemetery Tim -- who has also done headstones for Nate Dogg, Eazy E and Anthony Johnson -- to make the custom piece.