Chase Diggs and Rachel Robinson pulled up to the McDonald's drive-through window in Glen Burnie last October looking for a quick breakfast.

They ordered and paid, but then noticed through the window that three masked gunmen were taking everyone in the Cromwell Station restaurant hostage. When they saw an elderly man collapse and a gunmen run outside to a stolen van, Diggs and Robinson decided to leave their food at the window and follow.

"I thought, 'They're gonna shoot us,' " Diggs said.

They followed the van at a distance for nine miles to Federal Hill in Baltimore, and talked to police on their cell phone to let them...