South Portland Middle School was evacuated for the second time this month because of a gas smell in the building.

Students and staff were evacuated for roughly 20 minutes on Tuesday morning because of a gas odor in a mechanical room of the school, Superintendent Timothy Matheney said in a phone interview.

Matheney said he went to the school, smelled the odor and ordered the evacuation. He said the fire department arrived and did an air quality check, and determined there was no gas leak.

According to messages from the school’s principal to parents, the evacuation began around 8:12 a.m. and students were heading back to the school by 8:38 a.m. after the fire department had cleared the building. Students were evacuated to either Skillin Elementary School or South Portland High School.

A similar evacuation happened at the school on Jan. 6, when a gas odor was detected in two locations at the school, including a mechanical room. Then, students were evacuated for about an hour, and the fire department did not find any leaks.

Matheney said district officials believe the smell Tuesday was the result of a part malfunction with the school’s natural gas system. He said the malfunctioning part is currently being attended to, and said officials believe the odors in this incident and the previous one were both connected to the same system in the school.

The district is now doing a study of the larger system too root out problems, but Matheney said he is certain the system is functioning safely.

“We had a staff member report the odor, we took it seriously, we involved the gas company and fire department and we ensured staff members and students were safe at all times,” Matheney said.