Deputies responded to a home at about 3:56 a.m. to the 5800-block of Wild Plum Road in the Prairie Ridge subdivision after a report of a household member shooting their relatives.
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After arriving on the scene, deputies found three female victims were dead, another seriously wounded and a male seriously wounded.
Deputies said the wounded male was transported to a hospital, where he died. They said he was the aggressor in the incident.
The injured female victim was the caller and was transported to a hospital, the sheriff's office said.
"There really aren't words for something that tragic. It's tragic to see these people lose their lives," said neighbor Lynne Leon.
The sheriff's office said they were all members of the same household. Authorities did not provide further details on the ages or relationships of those involved.
McHenry deputies give update on unincorporated Crystal Lake shooting
Detectives are still trying to piece together what happened with neighbors shaken up by the violence.
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"This is our family," neighbor Holly Cote said. "I work at the school over here all these kids go to my school. This is just...I'm in shock, kind of."
Leon said she never saw her neighbors much; they kept to themselves.
"I only saw the ladies," she said. "There was at least two, but I think more than that. Three, four."
The victims have not yet been identified. An update from McHenry County investigators is expected Thursday morning.
Authorities said it is an isolated incident and there is no danger to the public.
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