In a final order, U.S. regulators assessed a $2.6 million civil penalty to Exxon Mobil Pipeline Co. for a 2013 oil spill.

About 3,100 barrels, down from the original estimate of 5,000 barrels, of a diluted form of heavy Canadian crude oil spilled from a 22-foot rupture in the Pegasus pipeline in Mayflower, Ark., in March 2013.

The federal Pipeline and Hazardous Materials Safety Administration said it concluded its study into the incident and assessed a civil penalty of $2.63 million to Exxon for violating federal pipeline safety regulations.

"The failure investigation concluded that the cause of the incident was a result of time intensified defects of originally manufactured pipe," PHMSA said in a statement.

A 243-page report published by the PHMSA in late 2013 said it was "highly probable" small cracks occurred when the original pipe was built in the 1940s. These cracks then would have spread and merged to form larger faults, which may have contributed to the rupture.

The PHMSA ordered Exxon to improve its failure detection system in order to identify weld seams that may be prone to failure. State regulators said Exxon should have known about the potential for failure.

The federal report said it took about 16 minutes for operators to isolate the situation after the failure was detected.

The majority of the 853-mile pipeline is operational, though the segment running though Mayflower remains out of service under orders from the PHMSA.