A decision on whether to bring charges could still be months away. The Minnesota Bureau of Criminal Apprehension is still investigating the case, agency officials said Friday. After they finish, Mr. Choi must choose whether to decide on charges himself or present the evidence to a grand jury, as he has done in past police shooting cases.

Speaking alongside Mr. Choi at a news conference in downtown St. Paul, Mr. Lewis, who is black, said that he was committed to considering the evidence carefully and that he hoped his work would instill confidence in the community. He also spoke of the emotions surrounding the death of Mr. Castile, a school cafeteria supervisor with an extensive history of traffic violations but no serious criminal record.

“We all understand the anguish and outrage that many of our residents feel about the loss of yet another black man in an encounter with a police officer,” Mr. Lewis said. “We all see how this incident and others like it around the country have caused deep divisions within our society.”

Glenda Hatchett, a lawyer for the Castile family, said Friday that she was pleased with the appointment of Mr. Lewis even though the family’s preference was for a special prosecutor to assume complete control of the case.

“Ultimately, what the family wants and what I am advocating on their behalf is that justice be served,” Ms. Hatchett said. Mr. Lewis’s appointment, she added, is “a strong step in the right direction.”