Chilean Toilet Paper Economics

For the past ten years, CMPC (a large Chilean multinational paper Co.) and a smaller Swedish rival controlled over 90% of all toilet paper sales in Chile. Apparently, they cooperated over these years to profit maximize. It is estimated that Chileans paid $500 million (roughly $3 extra per Chilean per year) more than what they would have had the market been competitive over these 10 years.

Explain using market structures, elasticity, graphs and economic concepts how these two companies may have sustained this price gauging for such a long time?