Ebele Okobi’s son was only three days into his life when a nurse at the hospital began making predictions. He was so big, the nurse said, that he was going to become a football player.

At that moment, Ms. Okobi, 44, said she knew she had to get out of America, a country in which being big and black often meant being at a greater risk of having a fatal encounter with law enforcement. “They’re already talking about him as if he’s big and intimidating,” she recalled thinking. “I can’t raise a black son in America. I don’t have that kind of fortitude.”

Ms. Okobi left for London four years ago with her husband and three children, finding a comfort she felt was elusive at home. But two weeks ago she received a call from her mother. Ms. Okobi’s youngest brother, Chinedu Valentine Okobi, 36, had gotten into a struggle with police officers in the San Francisco Bay Area. He was hit with a stun gun, and died.

“There was something about that call that felt inevitable, because it was something that I was running away from,” Ms. Okobi said.