On Saturday, Williams, 25, was charged with triple murder and according to charging documents, he confessed.
Officers said they do not yet have a motive for the killing. He's charged in connection with the deaths of his sister, six-year-old Nadira Withers of Brooke Jane Drive, as well as nine-year-old Ariana Decree and six-year-old Ajayah Decree.
Prince George's County police said they received a call from a home in the 6400 block of Brooke Jane Drive in Clinton, Maryland, at around 7:30 a.m. Friday after a woman found three young girls stabbed in a bed in a basement bedroom.
One of the girls was his 6-year-old sister, Nadira Withers. They were pronounced dead on the scene.
The DeCree sisters, who were the daughters of Kelley's cousin, were from Newark, New Jersey, and had been staying in Maryland for the summer.
Nadira Withers,9 victim
A man has been arrested in connection with the murders of three girls under the age of 10 who were found in a home in Clinton, Washington, DC on Friday. "The preliminary investigation reveals Williams had sole care and custody of the children at the home overnight into Friday", police said in a news release. He's being held without bond.
"This is one of the most hard scenes that our officers arrived on". 'We have not determined who the person responsible is yet, ' she said. "We have counselling services here at the scene dealing with first arriving officers", Donelan said, adding the neighbourhood is normally quiet.
Prince George's County Police Assistant Chief Hector Velez said the entire force was mourning the tragic loss of three young lives.
State's Attorney Angela Alsobrooks told NBC that Prince George's County had awoken to "yet another nightmare".