Police said their bodies were discovered around 7:30 a.m. Friday in a home in the 6400 block of Brooke Jane Drive in Clinton, Maryland.
Antonio Williams, 24, was arrested in connection with the murder of the three girls.
PEOPLE reports that Antonio Williams, 25, was arrested in connection with the deaths of three children: his 6-year-old sister Nadira Withers, and his two cousins, Ariana Decree, 9, and Ajayah Decree, 6.
"Detectives are working to establish a motive in this case", a police spokesman said.
Police say the Decree sisters were visiting from Newark, New Jersey.
On Saturday, Williams, 25, was charged with triple murder and according to charging documents, he confessed.
Blount County Animal Center participating in national adoption event on Saturday
Clear the Shelters is an initiative spearheaded by NBCUniversal Owned Television Stations, a division of NBCUniversal. The Waco shelter was not the only central Texas organization that took part in Clear the Shelters day.
When she returned home on Friday morning she found the girls in a basement room with apparent stab wounds, the fourth girl (2) was unharmed in the incident.
Williams is in custody of the Department of Corrections on a no-bond status.
The three girls all had stab wounds, and were pronounced dead at the scene.
In a Friday interview, Donelan hesitated to disclose details about the investigation, saying that she didn't want to jeopardize it. Prince George County Police Chief Hector Velez noted, "I wear a uniform, but I am a father". He added that they believed they'd made contact with a parent, but couldn't say any more than that. Those who responded here today are parents, aunts, uncles and siblings of young children who they care for and love very much.
"We are so very, very devastated for these young, attractive children and their families", Alsobrooks said. "We have counseling services here on scene dealing with our first-arriving officers".
"They are not alone in their grief". 'We have not determined who the person responsible is yet, ' she said.