Who killed tessa in the constant gardener?

In "The Constant Gardener" by John le Carré, Tessa Quayle is killed by a group of corrupt officials and agents involved in unethical pharmaceutical practices. Her murder is orchestrated to silence her after she uncovers evidence of the company's malpractice while working in Africa. The protagonist, Justin Quayle, seeks to uncover the truth behind her death, leading him into a web of conspiracy and corruption.