A man detained by police who were investigating reports of an assault died later in hospital.
Bedfordshire Police was called to Caddington, near Luton, at about 19:30 BST on Sunday after reports that a man had been attacked at a house.
Officers said a man in his 20s was detained in the house on Manor Road. He then became unwell and was taken to hospital, where he died.
The incident has been referred to the Independent Office for Police Conduct.
Officers remain at the scene while inquiries continue.
The police have not issued any more details about the attack on the other man which they were originally called out to investigate.