Elena Katherine Moore's cause of death is about to be announced by local authorities in South Carolina ... and TMZ is streaming live.

A spokesperson for the Lexington County Coroner's office announced that officials will reveal the cause and manner of the personal trainer's death. You may remember, Elena was found dead in June about a week after she went missing.

Elena disappeared after leaving a Planet Fitness and heading toward a wooded area. Her husband reported her missing the following day.

Days later, authorities found a body wearing clothes similar to Elena's ... and they later determined it was her. She was 39 years old.

An autopsy was completed shortly after her death ... which investigators say revealed no bruising, cuts, gunshot wounds, stab wounds, blunt-force trauma, strangulation, or other external injuries.

We spoke to a friend of Elena's, who says she was acting paranoid in the weeks leading up to her death. She also shared a series of cryptic posts that seemed to indicate she was having issues with someone in her life.

Another friend, Mendy Miller, told us she was well aware Elena had been on a fitness and health journey that included ridding herself of "toxic" people in her life, including friends.