OMAHA, Neb. (AP) — The Fort Calhoun nuclear power plant is back online less than a week after its main generator unexpectedly shut down as a precaution.

The Omaha Public Power District that owns the plant about 20 miles north of Omaha says it resumed generating electricity early Monday.

The plant shut down last Wednesday after a generator lockout.

OPPD says it believes the shutdown happened because moisture invaded a transformer control cabinet at the plant, but the incident is still being investigated.

While the plant was offline, workers performed some minor maintenance.

Fort Calhoun was shut down from April 2011 until December 2013. It initially shut down for routine maintenance, but significant flooding in 2011, a small fire and a series of safety violations forced it to remain closed for an extended period.