The investigation into alleged cyberbullying tied to Darrell Sheets' death is still actively underway, even after officials confirmed he died by suicide ... TMZ has learned.

The Lake Havasu City Police Department tells TMZ ... they're continuing to investigate allegations the "Storage Wars" star was targeted online in the lead up to his death. Cops tell us their Criminal Investigations Unit has already submitted Sheets' cellphone for forensic analysis ... a key step in determining whether any online harassment may have played a role in his death.

We broke the story ... the Mohave County Medical Examiner's Office told us Tuesday they've officially ruled Sheets' manner of death a suicide, pending toxicology results.

As we first reported ... Sheets was found dead last month at his Arizona home. At the time, officials said he appeared to have died from a self-inflicted gunshot wound to the head.

Now, with the cause of death confirmed, the focus is shifting to what may have led up to the tragedy.

Sheets' fellow "Storage Wars" star Rene Nezhoda has claimed Darrell was dealing with relentless online bullying before his death ... allegations police confirm are part of the active investigation.

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