SAN FRANCISCO — In the clearest sign yet that the White House will nominate Melinda Haag to be its U.S. attorney in San Francisco, the FBI has begun a background check of the Orrick, Herrington & Sutcliffe partner.

The Justice Department routinely vets administration picks for U.S. attorney before a nomination is formally announced and submitted to the Senate. Agents began conducting their background check of Haag within the past week, say two lawyers who were contacted by the government.