ST. PAUL — Demonstrators angered by the fatal police shooting of a black man during a suburban traffic stop kept vigil outside the governor’s mansion here Friday as officials urged calm and more details emerged about the officer who fired the shots.

Officer Jeronimo Yanez, who was placed on administrative leave after the killing on Wednesday night of the driver, Philando Castile, was a member of the St. Anthony police for four years. He had earned a bachelor’s degree in law enforcement in 2010 and was honored by his college as a top student. In the years since graduating, he had posted online about a wedding and the birth of a child and settled into a suburban neighborhood.

“He always wanted to dig deeper — what if this happened, or that happened,” said Christian Dobratz, one of Officer Yanez’s professors at Minnesota State University, Mankato. “I knew he was very big on wanting to work with others and serving a community,” he added.

But even as Officer Yanez’s background became clearer, the specifics of the shooting that left Mr. Castile dead remained murky.

The Minnesota Bureau of Criminal Apprehension, which is investigating, has said little about why Officer Yanez and a colleague pulled Mr. Castile over on a stretch of suburban road near the state fairgrounds, or what led to the shooting. Much of what is known comes from a Facebook Live video by Mr. Castile’s girlfriend showing the emotional, gruesome aftermath.

As that footage went viral online, protests have continued almost nonstop in St. Paul and the surrounding area, with activists calling for charges against Officer Yanez and a separate, federal investigation.