RED WING (AP) - Operators of the Prairie Island nuclear plant in Red Wing have shut down a unit because of a fire alarm.

Xcel Energy says there's no indication of a fire, but Unit 2 was taken off line about 4 a.m. Thursday so investigators can determine why the alarm was tripped.

Under rules established by the U.S. Nuclear Regulatory Commission, the operators declare a "notification of an unusual event." It's the lowest of four emergency classifications established by the commission.

Xcel says there's no threat to public safety.