A mother who carried her "unresponsive" two-year-old into a police station today was arrested on suspicion of his murder, police said.
The boy was taken by ambulance from Peterlee police station, County Durham, to Sunderland Royal Hospital, where he was confirmed dead shortly after arrival.
Due to injuries found on his body, the mother, 39, was arrested and held at Peterlee, Durham Police said.
The woman, from the Castle Eden area, was believed to have parked a Skoda Octavia outside the magistrates' courts which adjoin the police station.
The vehicle has been taken away for forensic analysis.
She walked into the modern police station's reception area between 7.30am and 8am, Durham Police said, carrying her son in her arms.
A spokesman said: "The child was not responsive and an ambulance was immediately called for.
"He was then rushed to Sunderland Royal Hospital where, despite the efforts of doctors, he was confirmed dead shortly after arrival.
"Due to injuries on the child's body, the 39-year-old woman was arrested on suspicion of the boy's murder, and she is now in custody at Peterlee police station."
A post-mortem examination was due to take place later to establish the cause of death.
Durham Police said the child would not be named at this stage.
The spokesman said: "A team of officers, led by Detective Superintendent Adrian Green, is working on the case and an incident room has been set up at the police station."
Officers cordoned off the front of the police station with tape and placed yellow cones outside the front door.
A North East Ambulance Service spokeswoman said: "We arrived on the scene within four minutes.
"The child was unresponsive at the time."
The ambulance took the child to the Sunderland hospital's accident and emergency unit.