The amount of the prescription painkiller Oxycodone found in the 2-year-old's blood was triple the level that would kill the average adult, officials said.
On Tuesday, the boy's father, Coco Kollie Wallace, 38, of Middletown Township, was charged with criminal homicide in the death of Sebastian Wallace and ordered held without bail at the Bucks County Prison.
At a news conference in Middletown, officials said three witnesses had affirmed that the father had Oxycodone in his possession when Sebastian Wallace somehow ingested the fatal dose last month.
Initially, Coco Wallace denied having the pills, according to a criminal complaint, because " 'I didn't want to bring suspicion to myself.' "
He later acknowledged having the pills, and a female companion told investigators that he had thrown away about $1,000 worth of Oxycodone, which he had acquired illegally, the day after the boy died.
Neither District Attorney David Heckler nor other officials at the news conference would say how the child ended up with the drugs in his system.
Sebastian Wallace died Oct. 22 in the emergency room of Lower Bucks Hospital in Bristol Township.
Toxicology tests affirmed the lethal amounts of the drug in his blood. To reach that level of toxicity would have required about six pills, said Ian Hood, the county medical examiner.
The father was the only person caring for Sebastian in the hours leading up to his death, police said. The mother lives in North Carolina.
A man who resides in the same apartment complex as Wallace and asked not to be named said he had found the incident hard to believe. He described Coco Wallace as an attentive father to the boy and a 9-year-old daughter.
"He ain't a man who neglected his kids," he said. As for the boy, he "was full of energy," but Wallace "kept him right in sight," the man said. "He was so strict on that little kid."