A health worker has been arrested on suspicion of administering poison with intent to endanger life after a child died at Birmingham Children's Hospital.
The 27-year-old woman was arrested on Thursday, hours after the death, and has since been suspended from her role at the hospital.
A spokesperson for the hospital said the child was being treated in the Paediatric Intensive Care Unit.
The woman has been released while investigations continue.
Forensic tests are also being examined, West Midlands Police said.
"The woman was arrested on Thursday evening at a property in the West Midlands area on suspicion of administering poison with intent to endanger life," a spokesperson for the force said.
"The family of the child are being supported by specially trained officers."
A spokesperson for Birmingham Women's and Children's NHS Foundation Trust said it was "supporting the infant's family at this distressing time and ask that privacy is respected during this process".
"Following the death of an infant at our Paediatric Intensive Care Unit at Birmingham Children's Hospital, we have asked West Midlands Police to examine what has happened, in line with our own safeguarding policy," it added.
"The staff member involved has been suspended by the trust following the national process on the sudden unexpected death of a child."
Birmingham Children's Hospital is a leading centre for specialist paediatric care and treats children and young people up to the age of 16.