SCRIBA -- Power output from an upstate New York nuclear reactor was reduced when off-site electricity to the plant was lost.

The U.S. Nuclear Regulatory Commission says the incident Tuesday at the Nine Mile Point Unit 1 reactor is an unusual event, the lowest of its four emergency classifications.

The Lake Ontario plant has two incoming power lines, one of which was shut down for maintenance. Officials are trying to learn why the second stopped providing power just before 8:30 a.m.

Diesel generators at the plant kicked in, providing emergency power while operators reduced output to 90 percent of normal. Full power was restored later in the morning and the unusual event was declared over at 10:22. a.m.