Six county jail inmates in north Alabama were taken to the hospital Friday after they ingested an “unknown substance.”

The Lawrence County jail inmates allegedly attempted to get high when they became ill, the county sheriff’s office told WAFF.

The inmates were drug screened at the hospital. No illegal narcotics showed up on the tests, the office told WAFF.

Four of the inmates returned back to the jail while the remaining two were expected to be back at the lockup later Friday.

The sheriff’s office was investigating how the substance made its way into the jail.