Ruiz admitted that he killed Dolci Gryshayeva because he was frustrated that she wouldn't stop crying, so he threw her to the ground and fell on her.
Ruiz was the boyfriend of Dolci's mother at the time.
He faces sentencing February 26th. The crime carries a penalty range of 25 to 40 years in prison.
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Ruiz, Dolci and her mother were all living in an apartment at 750 S. Vance St. in Lakewood when Dolci died in September of 2011.
A news release from the Jefferson County District Attorney's office says Ruiz admitted that "he snapped" when the girl wouldn't stop crying, and she hit the floor head first. When she became unresponsive, Ruiz called 911.
She died later at Children's Hospital from closed head injuries.