Scott Banda says in 1961, someone put "a wrong piece of pipe" into a 70-metre-long stretch of pipe at the facility.
He says the line was covered in insulation so the mistake wasn't visible, and wasn't detected in any of the several inspections that have happened over the years.
Co-op was fined $280,000 earlier this week for a safety violation under the Occupational Health and Safety Act after it was determined the cause of the blast was a corroded pipe.
Banda says they had been revamping the area and that section of pipe was just weeks away from being replaced when the explosion happened.
Since 2011, Banda says the company has done some reflection and has put a renewed emphasis on its internal safety culture so a similar blast never happens again.
"When you have that kind of an event at one of your facilities, it rocks you."
There has been at least one more explosion and two fires at the Co-op refinery since the October 2011 blast.
A blaze in May 2012 began in a pump used to move crude oil, a vent plug failed caused a fire damaging buildings and equipment in February 2013, and a frozen line caused a blast that resulted in $77 million damage on Dec. 24, 2013.