EXCLUSIVE

The mother of one of James Holmes' massacre victims has hired a high-powered attorney to sue the Aurora, CO movie theater where he opened fire ... killing 12 people, and injuring 58 others.

TMZ has learned Shirley Wygal is retaining a lawyer in Los Angeles to handle her case -- and we're told she plans to go after the Century 16 Theater.

Shirley's 32-year-old daughter Rebecca Wingo was shot and killed in the theater.

Sources close to Shirley tell us she believes the theater's emergency exit doors should have been alarmed, or there should have been security guards posted next to them ... especially for a premiere as anticipated as "The Dark Knight Rises."

Witnesses say Holmes entered the theater through an emergency exit door and then started shooting. Authorities think he may have propped the door open earlier from inside the theater.

TMZ broke the story ... a survivor of the shooting spree is planning a similar suit, but he also intends to go after Warner Bros. and Holmes' doctors.

We've reached out to Cinemark -- the theater's corporate owners -- but no reply yet.