A container ship lost power and rammed into Baltimore's Francis Scott Key bridge early Tuesday morning, causing the span to buckle into the river below and plunging a construction crew and several vehicles into the dangerously cold waters. Rescuers pulled out two people, but six others were missing.   The ship's crew issued a mayday call moments before the crash took down the bridge, enabling authorities to limit vehicle traffic on the span.  "These people are heroes," Maryland Gov. Wes Moore said. "They saved lives last night."  Maryland transportation officials reviewed traffic cameras and now confirm there were “no vehicles transiting the bridge at the time of the incident,” according to the latest internal DHS briefing obtained by ABC News.  The pilot who as at the controls “is currently undergoing post-accident Drug and Alcohol Testing,” the briefing said.