Following his conviction for second-degree murder, Walter Mousseau Jr., 30, of Pine Ridge, is now undergoing a separate trial to determine if he was legally insane at the time of the July 4, 2022 drowning of Sheldon Glenn, 43, in Rapid City’s Memorial Park Pond.
Mousseau was arrested the same day Glenn drowned during an Independence Day concert. A Pennington County jury found Mousseau guilty of second-degree murder and unauthorized ingestion of a controlled substance, but acquitted him of first-degree murder. He had pleaded not guilty by reason of insanity.
Defense attorney Martha Rossiter argued Mousseau suffered from untreated schizophrenia and a traumatic past, which distorted his perception and led him to believe he was defending himself when he drowned Glenn. Witnesses did not see how the men entered the water.
Prosecutor Emily Toms countered that while Mousseau may have a mental illness, it does not equate to legal insanity. She said the evidence would show he acted knowingly and has a history of substance abuse and manipulating his mental health.
The insanity phase of the trial is scheduled to run through Friday.
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