() The parents of a 10-year-old California boy who died from fentanyl poisoning say they believe their son may have been intentionally given the deadly drug, and they’re demanding stronger charges against the woman accused in his death.

Marcy Castro and Miguel Mendoza told ’s “Banfield” that they suspect Christina Alvarez, the girlfriend of Nathaniel Castro Mendoza’s uncle, deliberately poisoned their son in December. The couple cited the fact that multiple people lived in the house but only their son was affected.

“In my heart, I feel like maybe it was done purposely. Obviously, we can’t prove that,” Castro said Monday. “The only reason we feel that way is there was many other people in the household. And for some reason, only our son was affected by it.”

Nathaniel, who loved baseball, pro wrestling and “Captain America,” stopped breathing without apparent reason after eating dinner at his home outside Los Angeles in December. An autopsy found fentanyl and another drug in his system.

Alvarez was arrested in March and charged with felony child endangerment with an enhancement for willful harm resulting in death. Police said she “possessed and stored fentanyl in a manner that led to the victim’s death.”

The parents expressed frustration that prosecutors did not file murder charges. Miguel Mendoza said prosecutors explained “they weren’t able to prove certain things as far as the murder charge” and that child abuse with enhancement was “the best they can do here in California.”

Castro described her son’s final evening as seeming normal. Nathaniel came home from school, played outside, rode his scooter and ate shrimp that Alvarez had brought home from a restaurant. He went to bed and woke up once around 11 p.m. to use the bathroom before going back to sleep.

“Unfortunately, that was the last time Nathaniel was alive,” Castro said.

The parents questioned how only Nathaniel was poisoned when Alvarez claimed she and her two children also ate the same shrimp dinner. Alvarez had been out all day and brought the food home from a restaurant, according to Castro.

The family has launched a “Demand Justice for Nathaniel” movement that had garnered about 2,000 signatures. Castro said they want to “raise awareness, educate, advocate, protect” and “support families with similar tragedies and fight for accountability.”

Fentanyl is a legal opioid that is 50 times stronger than heroin but is frequently abused and has become a leading cause of overdose deaths nationwide.