A Virginia woman was arrested Tuesday, days after she was charged in the death of her 20-month-old son, officials said.

The Prince William County Police Department said Tiffany Nicole Stokes, 37, turned herself in after she was formally charged last week after it was learned her son ingested a lethal dose of fentanyl. 

She was charged with felony homicide, felony child neglect and possession of a scheduled I or II narcotic, authorities previously announced.

The arrest culminates a monthlong investigation into the child’s mysterious death after he was initially found dead in June. 

VIRGINIA MOTHER CHARGED WITH HOMICIDE IN FENTANYL-RELATED DEATH OF 20-MONTH-OLD SON

Prince William County police said Stokes woke at about 3 a.m. on June 23 in an apartment on Soapstone Drive in Gainesville and found her son unconscious, lying in bed. 

The child was rushed to the hospital, where he later died. 

No foul play was initially suspected, but a medical examination in August found fentanyl in the boy’s system. The medical examiner determined it caused his death. 

DOCTORS WARN FENTANYL BECOMING STRONGER, MAKING REVERSING OVERDOSES MORE CHALLENGING

Investigators determined the fentanyl was ingested by the child sometime during the evening of June 22 or early the next morning, police said.

The new detail prompted police to question Stokes, who was allegedly found in possession of unprescribed pills containing Oxycodone and Acetaminophen. 

She is believed to have had access to and used the unprescribed pills at the time of her son's death.

An arrest warrant was issued for Stokes Wednesday, Dec. 7.

Fox News' Louis Casiano contributed to this report.