At midnight, staff at Cardon Children's Medical Center checked on a 5-year-old patient and his mother, who was staying with him at the Mesa, Arizona, hospital.
Both were there, all appeared well.
But just two hours later, in the early hours of Saturday, during the next check, the child and his mother were found dead, police said.
Mesa police spokesman Steve Berry called the nightmarish scene an apparent murder-suicide. The mother, who was in her 20s, apparently fatally shot herself. Berry did not say how her son died.
The child had been in the 206-bed children's hospital, which sits alongside Banner Desert Medical Center some 15 miles east of Phoenix, for "about a week," Berry said. He was in "a regular room," the police spokesman added. Authorities did not reveal the reason for the child's hospitalization.
The Arizona Department of Child Safety was contacted about the situation as recently as Thursday, according to the police spokesman, who did not say why.
David Lozano -- a spokesman for Banner Health, the company that runs Cardon Children's Medical Center -- said that "at no time were others impacted" by what he also called the "apparent murder-suicide."
"The safety of our patients, associates and visitors is paramount to us and is our top priority at this time," Lozano added.
It's not clear how the child's mother got a gun into the hospital. Banner Health has a policy -- which is posted at entryways -- against carrying firearms and weapons into its facilities, Lozano said.
Berry told CNN affiliate KXNV that the hospital is "cooperating in any way possible" with the investigation.
"They will provide our investigators with information on the child, the family, ... any treatment, anything that may assist us, if you can -- if there's any way to understand -- what led to this."