An attorney for Mingdong Chen didn't disclose the 25-year-old's diagnosis Monday but said he's exhibiting symptoms of severe mental illness that prevent him from making reasoned decisions.
Lawyer Danielle Eaddy says Chen will be brought to an inpatient psychiatric facility.
Chen faces five counts of murder in the stabbings of his cousin's wife and her children in Brooklyn's Sunset Park neighborhood last October. He's pleaded not guilty.
The mother was 37-year-old Qiao Zhen Li and her children.
The children were 9-year-old Linda Zhuo; 7-year-old Amy; 5-year-old Kevin; and 18-month-old William.