Union Pacific continues to investigate the cause of a derailment Friday morning in Tucson.

A freight train was traveling from Nogales to Tucson when eight empty cars derailed near East 16th Street and South Euclid Avenue, Aaron Hunt, a California-based spokesman for Union Pacific, said in an email.

The derailment, which took place around 6 a.m., closed roads in midtown and downtown Tucson until about noon, forcing rush-hour drivers to find alternate routes from 16th Street and South Toole Avenue to East 22nd Street near Santa Rita Park, said Sgt. Chris Widmer, Tucson Police Department spokesman.

No one was injured, said Capt. Barrett Baker, Tucson Fire Department spokesman.

No other trains were delayed because of the derailment, Hunt said.