Detectives in Lancaster say Diedre Allen, 49, burst into a home in Lancaster on Friday wearing an old man mask, brandishing a pistol and poison, and carrying a recorded manual for murder.
Investigators say Allen planned to murder a mother and her 3-year old son, and make it look like a murder suicide.
But, she was wrestled to the ground by family members who held her until police arrived. Investigators believe the woman may have had a relationship with her intended victim's husband.
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Friday night, deputies said Allen was at least six months pregnant, a fact that was first discovered by one of the family members who wrestled her to the ground.
"He got her in a choke lock," said Eduardo Vasquez, one of the relatives. "He didn't know it was a woman until he took the mask off and realized it was a pregnant woman."
Deputies say the recording provided important details about the plot.
"It sounded like she was going to potentially smother the toddler and give the mother a poisonous type of liquid," said Sgt. Robert Rush of the L.A. County Sheriff's Department. "It was very bizarre. If you listen to that tape, it was pretty chilling."