Nikola was the buzz of Wall Street, trying to cut a path in electric trucking. Now, federal prosecutors are investigating claims that it misled investors. WSJ explains Nikola’s roller-coaster summer and what’s next for the company. Photo Graphic: WSJ Auto makers have spent decades developing hydrogen fuel cells as a green alternative for cars with little to show for it. Now, they are shifting their attention to the trucking industry. The technology, which mixes hydrogen with oxygen to create electricity, propels a vehicle’s motors and emits only water vapor. Car companies say it has particular advantages for commercial trucks that travel long distances and need to refuel quickly, and they are trying to satisfy growing interest in alternative-fuel big rigs. Tougher…