The woman's mother returned home in the 3100 block of Norwich Avenue to find her daughter unresponsive. Responding paramedics were unable to revive her. A coroner identified the woman as Monique Rivera.
The family dog was also found dead at the home.
Alhambra police say there was no evidence of a struggle. Neighbor Lidia Centeno says the woman was a nursing student at Cal State Los Angeles.
Officials say poisoning may have been caused by a water heater that wasn't vented properly.
An autopsy was scheduled for Tuesday.