The incident happened in September 2020 inside a mobile home in the 5400 block of 34th street in Jurupa Valley.
The Riverside County Sheriff's Department said the investigation was lengthy, but ultimately led to the arrest of 22-year-old Adler Metcalf and 20-year-old Sandy Acuna on Wednesday.
According to the arrest warrant, Acuna called 911 and told the dispatcher her son overdosed on Oxycontin. She told investigators that while she was sleeping, she believed her son crawled out of his crib and onto a counter where he swallowed a white pain pill.
According to the warrant, she later changed her story to say that she discovered the pill was missing and went to check on the baby.
Investigators said a search warrant was served on Acuna's cell phone, where text messages showed several photos of blue "M-30" pills, which are consistent with counterfeit pills that contain fentanyl.
Fentanyl is a synthetic opioid that's 100 times stronger than morphine, according to the sheriff's department.
The warrant stated the couple's other 4-month-old child tested positive for Cannabinoids and Carboxy-THC.
Investigators believe both parents were giving fentanyl and marijuana to their children.
"It is horribly tragic," said Riverside County Sheriff Chad Bianco, who said many fentanyl overdose victims are children. "It's a tragic situation where this family was obviously involved with drugs, where the parents were involved with drugs, and allegations of possibly selling drugs and their kids are paying the price."
At a recent news conference, Riverside County District Attorney Mike Hestrin announced he would be seeking murder charges against drug dealers if one of their customers dies from a fentanyl overdose, calling the epidemic "fentanyl poisoning."
"In Riverside County, in 2016, there were two fentanyl deaths," said Hestrin. "This year, we're on pace to have between 500 and 600 fentanyl-related deaths."
Bianco estimates that perhaps half of the fentanyl-related deaths that have occurred in Riverside County happened to people who did not know the risk they were actually taking.
He said it's a growing problem that people need to take more seriously.
"When you use the word 'overdose,' our brain is programmed to believe that they did it to themselves," he said. "That they're a drug dealer, they took too much, and they knew the risks, but the problem with fentanyl is that it's so potent. The dose of fentanyl could be like the size of tiny grain of salt. If two happen to get in that one pill, you're dead, and there's no bringing you back."
Metcalf and Acuna are both facing one count of murder and several other charges, including willful harm to a child.
They're scheduled to be arraigned next week.