Michigan Murder-Suicide: Three Dogs and One Cat Also Died in House Fire

The Michigan murder-suicide not only saw the shooting deaths of two parents and six children, but also the gruesome deaths of a bunch of pets who were found inside the family's "house of horror" ... TMZ has learned.

Ottawa County Sheriff's Office Captain Jake Sparks tells TMZ ... after police and firefighters responded to the Karolkiewicz house on July 24, they found at least 3 dead dogs and one dead cat inside the home. The pets all died from smoke or fire, and there may have been more pets who perished.

Sparks also said the bodies of Kristopher Karolkiewicz, 47, and his wife, Amanda \u201cMandy\u201d Karolkiewicz, 39, were discovered in their bedroom. The couple's 4 sons and twin daughters -- ranging in age from 5 to 11 -- were found in their bedrooms as well.

The captain said Kristopher used a semi-automatic handgun to kill his family before setting the home on fire and turning the gun on himself. Sparks said Kristopher fired at least 8 shots during his rampage.

Sparks said investigators are combing the neighborhood for video and talking to witnesses, family and friends to piece together a timeline and to drill down on a motive.

Earlier this week, TMZ reported that Amanda apparently posted to Reddit messages about an affair Kristopher had years ago that "shattered" her life.