It's unclear how it happened but London police are investigating an incident where seven inmates at the Elgin-Middlesex Detention Centre overdosed at the same time.

Police, fire and EMS crews were called to the prison on Exeter Road Thursday around 2:15 p.m.

But seven prisoners from one range had to be transported to hospital.

As of 5 p.m., six people were in fair condition and one was in serious condition.

It's unknown what drug they ingested or how they obtained the narcotics.

Police say they will continue to investigate.