Rubio was convicted in 2003 of smothering, stabbing and mutilating his toddler son and infant daughter, along with his wife's 3-year-old.
The Texas Court of Criminal Appeals overturned Rubio's conviction last year because his defense did not get to cross-examine the common-law wife and co-defendant, Angela Camacho, during trial.
The Cameron County District Attorney's Office has subpoenaed interview notes, videos, letters and an unedited interview with Camacho from Weslaco television station KRGV. A reporter from the station interviewed Camacho in October 2007.
Camacho is serving a life sentence for her role in the killing of the three children -- Julissa Quesada, 3, John E. Rubio, 14 months, and Mary Jane Rubio, 2 months.