Though the United States government tried to cover up the story as a simple riot, this couldn't be further from the truth. The town was in fact under attack from zombies.
The zombies arose when Carlito Keyes unleashed the Ampulex Compressa Giganteus species on the town in revenge for a similar outbreak in the Central American town of Santa Cabeza accidentally caused by the US government, resulting in the death of his friends and family. Special Forces were forced to kill everybody in Santa Cabeza when they turned into zombies.
The town of Willamette soon crippled under the force of the undead. Bands of survivors retreated to the towns only source of entertainment; the Willamette Parkview Mall, and barricaded themselves inside. Carlito himself had set up a hideout within the mall, where he monitored the survivors, and kept tabs on activity outside the mall.
When news about Willamette being quarantined went public, idealistic photojournalist Frank West set off to cover the story and make a name for himself. However, when he arrived, the entrance to the mall was breached and the zombies flooded in.
Frank quickly discovered the real situation, and took it upon himself to solve the case behind the outbreak with the help of DHS agents Brad Garrison and Jessie McCarney. Another source of help came in the form of Carlito's younger sister; Isabela Keyes. Isabela shared her brother's grief for the people of Santa Cabeza, but was able to accept that the outbreak was an accident, and the intentions of the research conducted there were good.
After learning of the full truth, Frank and Isabela escaped Willamette and spread the truth about both incidents to the public. The US government were forced to admit responsibility for Santa Cabeza, but they blamed Willamette solely on terrorists (which, in a sense, was true, as Carlito's actions could be considered such.).
Since Frank and Isabela's escape, Willamette has been sterilized by the US military. The undead population of the town has been exterminated and the town has been sealed off indefinitely, devoid of any life.