TransCanada's (NYSE:TRP) Keystone pipeline is back in business, after safety concerns prompted its closure last Wednesday.

Grady Semmens, a TransCanada spokesman, told The Associated Press last week that the line was shut down so closer inspection could be made after the company "found a small anomaly on the outside of the pipe." 

Oil resumed flowing Monday afternoon, according to the AP, which quoted another company spokesman as saying "the integrity of the pipeline system is sound." 

The Keystone pipeline carries 590,000 barrels of crude oil per day from Canada to US refineries throughout the Midwest.

It stretches 2,100 miles, and is the backbone to the controversial $7 billion "Keystone XL" project, meant to expand the pipeline's reach from tar sands crude oil in Canada to Texas Gulf Coast refineries.