CLAYTON, Mo. — It was a summer evening 50 years ago when a knife-wielding kidnapper made the kind of malevolent move that defines a boy for life: The kidnapper snatched a police officer’s gun and used it to kill a canine officer, leaving 12-year-old Robert P. McCulloch without his father.

The boy would grow up with dreams of becoming a police officer, too, but a few years after his father’s death, those hopes were scuttled when cancer claimed his right leg. He went on to become the St. Louis County prosecutor instead.

After the killing of Michael Brown by an officer in Ferguson, Mo., Mr. McCulloch, the county’s top lawman for 24 years, is again facing the questions that have dogged his career for two decades: Can he be objective in cases involving black men and white police officers, when his own wounds run so deep? His father’s killing, deep family ties to the police and past entanglements with the black community have contributed to a wave of calls for his removal from a case that has gripped the nation.

Protesters angry over the killing of Mr. Brown, an unarmed black teenager, marched outside Mr. McCulloch’s office Wednesday demanding that he step down. At least 70 clergy members singing gospel hymns paraded to his office and read a letter asking, in part, that he recuse himself. A group of activists is expected to show up there again on Thursday with more than 70,000 signatures asking him to appoint a special prosecutor. A state senator wrote him a letter asking him to step aside, and even the governor, Jay Nixon, hinted that Mr. McCulloch, a fellow Democrat, should recuse himself.