"I am sorry that Courtney did that. ... First let me say I'm sorry to the Burk family for Courtney taking, taking their child. ... My heart goes out to her family," Williams said in the tearful interview.
But she also said her son did not confess anything to her.
Lockhart, 23, was arrested Friday in Phenix City, Ala., and was charged with capital murder in Burk's abduction and shooting death. Police would not say what led them to charge him in her death.
The 18-year-old from Marietta, Ga., was found shot on the side of an off-campus road Tuesday night and her car was found burning in a campus parking lot.
Williams told the television station that her son hasn't been the same after serving 16 months in Iraq. She says her son had been living with her in Smiths Station, Ala., since returning from the war.
The Burk family likely did not see the Saturday evening TV interview, family spokeswoman Kathy Singleton said.
"The family didn't watch any TV last night. I honestly don't know what their reaction is," Singleton said. "My heart breaks for anyone that's attached to the whole tragic situation. It's hard on everyone."
One number for Catherine Williams in Smiths Station was unlisted. No one immediately returned a message Sunday from The Associated Press at another listing for Cathy Williams in the same town.
