The lockout was put in place as a precautionary measure.
The decision in the Aug. 9 shooting had been expected, in part because of the high legal standard needed for a federal civil rights prosecution.
Justices appeared divided on ideological lines as the court heard a second major challenge to President Barack Obama's healthcare law.
At least five cows reportedly were killed and about a dozen others were injured.
Outages were first reported around 8 a.m.