Stockley fatally shot Anthony Lamar Smith in 2011. Stockley testified he thought Smith, who was black, was reaching for a gun. Prosecutors say Stockley planted the weapon inside Smith's car.
Activists have threatened civil disobedience if Stockley, who is white, is acquitted.
Media reports and preparation announcements from officials indicate that Judge Timothy Wilson may rule Friday. Gov. Eric Greitens has the National Guard on standby. Police are working 12-hour shifts starting Friday. All downtown courthouses will be closed, including the federal courthouse.
Greitens and Mayor Lyda Krewson called for peace. Smith's fiancee, Christina Wilson, asked that any protests be peaceful.