Further tests will need to be carried out on the body of a 15-year-old boy, found in a bath on Christmas Day, to determine an exact cause of death, police said.
Officers discovered the badly-injured teenager at a flat in Hathaway Crescent, Newham, east London, about lunchtime on Saturday, after being advised he had drowned in a bath.
A man and a woman, both aged 27, have since been arrested, with police treating the death as suspicious.
They remain in custody at separate east London police stations.
A post-mortem examination at East Ham Mortuary failed to determine the exact cause of death.
Police are now waiting for results of further tests.
A police spokesperson said: “At this very early stage, this is believed to be a domestic incident.”