Students have returned to in-person classes at a North Carolina middle school after the building underwent extensive cleaning to remove mold.

WCNC-TV reports that West Rowan Middle School in Salisbury, N.C., was closed on Aug. 22 after mold was found in the building’s HVAC system

Custodial staff reported microbial growth in the HVAC system earlier in August and the district began testing for mold and other contaminants.

Initial tests for mold back negative, but further test results indicated additional cleaning was necessary, so the Rowan-Salisbury district shut down the building, and students began taking their classes online.

Workers cleaned the system and retested it before students were allowed to return. Staff returned last week to prepare for the students’ return.

Testing for mold and other issues will continue throughout the school year.