A spokesperson for the Humboldt County Sheriff's Dept. tells TMZ ... cops got a call Monday morning just after 10 AM in Eureka, CA reporting an unexpected death at a home. We're told when deputies arrived they found Mike dead at the scene.

Cops say drugs and alcohol are not believed to be factors in the death and no controlled substances or paraphernalia were located at the scene. At the time of this post ... the cause of death is unknown.

The band released a statement on Facebook shortly after Mike's passing and said, "It is with our deepest regrets that we must announce the passing of our brother, our friend and true legend of heavy metal music."

The statement continued, "We are devastated and at a loss for words. Please respect our privacy and the Howe family’s privacy during this most difficult time."

Metal Church, universally credited with influencing the thrash metal genre, was formed in the 1980s and released 2 albums before Mike joined the group later that decade. He'd ultimately help pump out the band's third album ... "Blessing in Disguise," which churned out one of the group's most popular tracks, "Badlands."

The group would go on to release 2 more studio albums before the band broke up in the mid-1990s. The band ultimately got back together and Mike rejoined them in 2015.

Mike was 55.