Lydia Centeno was struggling to understand how her neighbor from across the street, a friendly 25-year-old woman with a promising career in nursing, died Monday after being found unconscious.
Centeno says Monique Rivera and her toy poodle were found unresponsive by Rivera's mother, who had returned to their Alhambra home from grocery shopping.
"At 9 o'clock in the morning she was alive. Two hours later (her mother) came back and she was dead," Centeno said.
Police were investigating whether carbon monoxide poisoning killed Rivera and the family dog. Inside the home, firefighters detected high levels of the odorless gas.
"Based on some of those instrument readings, the focus of the investigation right now is a water heater that possibly wasn't ventilated properly" said Alhambra Police Sgt. Jerry Johnson.
Police say the water heater was inside the kitchen, not in a garage or backyard where they're normally found.
"I'm just shocked," said Jaime Mora, another neighbor. "We actually have one of those detectors at my house. You can't tell whenever it hits."
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The official cause of her death had not yet been determined. An autopsy was scheduled for Wednesday.