London, Ont. police are investigating after seven inmates at the Elgin Middlesex Detention Centre overdosed on Thursday within minutes of each other.

Police, emergency and fire crews were called to the jail in the south end of the city at about 2:15 p.m. after correctional staff observed the inmates in distress. Medical staff at the jail also responded.

Police said the seven inmates who overdosed were conscious before being transferred to the London Health Sciences Centre where they remain. Officials said six inmates were in fair condition and one was in serious condition, as of 5 p.m. on Thursday.

Police said naloxone was used but it's unclear how many inmates were resuscitated using the life-saving medication.

The inmates who overdosed were among a group of 14 people who were seen together before the incident took place inside the jail, police said.

"All [affected] inmates were on the same range and appeared to overdose within minutes of each other. How it happened is unknown," said London Police Const. Michelle Romano.

It's unclear what drug was used and how the overdoses happened.

London police has launched a joint investigation with the Ministry of Community Safety and Correctional Services.