Investigators say a boat was attempting to set anchor, when something malfunctioned, causing it to drift.
The yacht then reversed to avoid another boat and ended up hitting a raft that was floating behind it. Two women who were on that raft were seriously hurt.
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In the new report the Illinois Department of Natural Resources said "improper lookout" was one of the reasons for the accident.
It also cites "operator inattention" and the captain operating the boat too close to another vessel or person in the water.
One of the two women injured, Lana Batochir, underwent a double amputation of her legs.
In a YouTube video from her hospital bed, Batochir said her children are what's keeping her going in her recovery.
She said she hopes to see her children soon.
No charges have been filed in the incident and an investigation is still underway.