The cast for film based on the real-life West Memphis Three keeps getting bigger: In addition to stars Reese Witherspoon and Colin Firth signing on, Deadline is reporting that The Killing star Mirielle Enos has joined the cast as a "mother of an 8-year-old boy who lives in a run-down house in West Memphis." Her character will reportedly become a key witness in the case against the West Memphis Three, a trio who becomes accused of killing of three 8-year-old boys. 

Witherspoon is set to portray the mother of one of the victims, and Firth a detective working on the case. The script is said to be based off of investigative reporter Mara Leveritt’s book Devil’s Knot: The True Story of the West Memphis Three, and was written by The Exorcism of Emily Rose writers Scott Derrickson and Paul Boardman.

[via Deadline]