PRETORIA, South Africa — The Olympic athlete Oscar Pistorius was declared not guilty of murder on Thursday when the judge in his case said she accepted the main argument in his defense: that he believed he was firing at an intruder rather than at his girlfriend, Reeva Steenkamp, when he killed her in a burst of gunfire through his bathroom door last year.

But then, after laying out why Mr. Pistorius, 27, might instead be guilty of a lesser crime, culpable homicide, the judge abruptly stopped in the middle of her explanation and suspended court without declaring a verdict on that charge.

“We’ll have to stop here and resume tomorrow morning,” the judge, Thokozile Matilda Masipa, said, less than half an hour after the court had returned from its lunch break.

Tears streamed down Mr. Pistorius’s face as he listened to the verdict and he began sobbing.

Mr. Pistorius, who was born without fibulas and had his lower legs amputated as an infant, was a national hero and an international star for the way that he overcame his disability to compete against able-bodied runners as well as disabled ones. But the sheen has worn off since the death of Ms. Steenkamp on Feb. 14, 2013, and some people said on Thursday that he had gotten off too lightly.