well, there's this, so it wasn't a completely silent event, although it doesn't specify the Mom - it states the shooter was leaning over three bodies at the same time, which indicates to me they were dead and not crying out - the Mom was found in the bathroom
She then heard someone shouting “stop” and “help” and what she believed was her 15-year-old brother speaking on the phone, according the probable cause document. She told police he was leaning over the three bodies in the hallway to see if they were still alive, then came back to the bedroom to check on her.
The teen's 11-year-old sister told King County detectives that she survived the mass shooting by playing dead, according to a court document.
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