The boy, charged with second-degree murder in with the stabbing death of his 8-year-old sister, entered the plea Wednesday in a Calaveras County juvenile court, CNN reported.
"I say he is innocent," the boy's attorney, Mark Reichel, told CNN. "We are going to come back on July 31 to set a date for a trial on that day and we are getting into the issue of whether a 12-year-old can ever be put on trial for murder."
Leila Fowler and her brother reportedly were alone April 27 in their family's Valley Springs, Calif., home when she was attacked, police said. The boy told police he saw an intruder leaving the home and found his sister suffering from stab wounds. She died soon after arriving at a hospital.
After searching the family's home and neighborhood, tracking leads and interviewing registered sex offenders, police arrested the boy May 11.
Reichel told CNN the boy has his family's support and is "doing OK under tough circumstances."