HENNEPIN, Ill. (AP) — Authorities are investigating what caused a transformer fire at a northern Illinois power plant that forced one of two power-generating units to shut down.

The (LaSalle) News-Tribune reports (http://bit.ly/1rL3pU1 ) that firefighters were called to the Dynegy Power Station in Hennepin around 11 a.m. Tuesday as thick black smoke rose from the site along the Illinois River. Smoke was no longer visible by noon.