CONCORD, N.C. (AP) — Seven high schools in a North Carolina system were evacuated and closed after an unspecified threat which officials determined wasn't credible.

All high schools in the Cabarrus County Schools system were closed on Thursday out of what police said was an abundance of caution. A school system spokesman said a middle school was also dismissed, but the county's two early college schools remained open.

The Concord Police Department and Cabarrus County Sheriff's Office conducted a joint investigation into the threat against the schools. Authorities declined to give details of the threat, except to say it made reference to explosive devices.

The schools received an all-clear around 1 p.m., but an investigation into the incident.