A 25-year-old has confessed to stabbing to death his 6-year-old sister and two young cousins while babysitting the family at their Maryland home, police said.

The three girls, all under 10, were found dead in a single bed with lacerations to their upper body, according to the Prince George's County Police Department.

Antonio Williams' mother found her daughter and the other two children in the basement of her home.

Williams' mother had asked her son to care for the children while she worked an overnight shift.

Williams has been charged with three counts of first and second-degree murder in the deaths of his sister, 6-year-old Nadira Withers, and two second cousins, 9-year-old Ariana Decree and 6-year-old Ajayah Decree.

Police said the Decree sisters were visiting from Newark, New Jersey. Their mother was also present when they were found, at 7:30 a.m. on Friday.

A fourth child, a 2-year-old, was also in the home at the time of the killings but was unharmed.

Williams was the only adult in the house at the time of the killings, said Tony Schartner, of the Prince George's County Police Department.

Police are still trying to determine a motive.