The incident occurred in Patterson's apartment, where the boy had recently moved with his mother. Patterson called 911 on Wednesday evening to report a choking at the Platinum Valley apartment, but Lt. Blaine Larsen of the Sioux Falls Police Department said it became clear at the hospital that the boy's injuries were not accidental.
Sioux Falls police said the incident was initially reported as a medical emergency.
"As officers and first responders got there, they found child was unresponsive and he was taken to the hospital," police spokesman Sam Clemens told USA TODAY Sports. "So they ended up calling police about that or letting them know. We had detectives talk to quite a few people to try to figure out what happened. What they ended up finding was that when the injuries happened there was one person with that child, and that person was Joseph Patterson."
"The injuries they found were head injuries," Clemens said. "It was obvious to medical staff that abuse had taken place and that those injuries weren't accidental."
Clemens said police believe the child's mother was "gone for a short time, and that's when we believe the injury occurred."