Who is Killing Colombian Environmentalists?

Setting a new record, Global Witness recently revealed that 227 environmental activists were killed last year. Since 2012, Global Witness has tracked murders of "environmental defenders" who protect land, water, and wildlife. Because attacks are underreported, Global Witness acknowledges that the number of fatalities may be higher. Looking at their latest report, for the second year in a row, Colombia is the most dangerous country in the world for environmental activists. Why are Colombian advocates the most vulnerable right now?