Explosion in Union Pacific's massive railyard in Nebraska appears accidental, investigators say

Investigators say explosion near Union Pacific's railyard in western Nebraska appears to be accidental.

Nebraska State Fire Marshal's office is working with railroad and experts from the Federal Railroad Administration .

Investigators haven't found any sign of a criminal act connected with the explosion, so the incident appears accidental.

Area of about a one-mile radius around where explosion happened was evacuated for several hours .