A city employee was killed outside Baltimore City Hall after a vehicle sped off an interstate and onto downtown streets, flipping the car he was driving as he tried to make a turn. (Christopher T. Assaf/Baltimore Sun video)

The investigation will involve an accident reconstruction to determine the speed and details of the crash, Bernstein said. A toxicology analysis will determine whether the driver was drunk or under the influence of drugs.

"You hope the state police will move expeditiously," Bernstein added. "We'll wait for the state police to complete its investigation."

Jim Crawford, a Baltimore defense attorney for more than 20 years who has handled vehicular homicide and manslaughter cases, said such investigations typically play out slowly and methodically.

Police are "going to defer in any type of case like this, normally to the state's attorney's office, because they don't want to screw it up," he said.

Warren A. Brown, a Baltimore defense attorney, said he has never seen a law enforcement agency immediately charge a driver in a vehicular manslaughter or homicide case.

"It takes a lot of investigation for them finally to decide," he said. "They'll bring charges but they'll have their house in order for that to occur."

For example, Thomas Meighan Jr., who was convicted in 2011 of hitting and killing Johns Hopkins University student Miriam Frankl, was not arrested until a week after she died.

At least one past case has shown charging drivers involved in deaths with traffic violations can endanger vehicular homicide prosecutions. In 2004, an Ellicott City woman avoided a vehicular-homicide charge after killing a motorcyclist in a drunken-driving accident because a police officer issued the woman a negligent-driving citation on the night of the crash.

A Howard County circuit judge ruled that the citation protected her from prosecution on a vehicular-homicide charge, because negligent driving is considered a lesser form of motor vehicle homicide.

Hersl's friends are struggling to imagine how the victim would have wanted the case to unfold.

"He was always fair, so I know he would want a fair process. I think he'd leave it to the state's attorney," friend and neighbor Paul Oliver said. "He believed in the system and he believed in doing things right. It's a tough one to swallow."

Baltimore Sun staff writer Alison Matas contributed to this article.