Two members of the FBI’s elite counterterrorism unit died Friday while practicing how to quickly drop from a helicopter to a ship using a rope, the FBI announced Monday in a statement. The statement gave few details regarding the deaths of Special Agents Christopher Lorek and Stephen Shaw, other than to say the helicopter encountered unspecified difficulties and the agents fell a “significant distance.” Last month, the team was involved in the arrest of Dzhokhar Tsarnaev, a suspect in the Boston Marathon bombings. And in February, it rescued a 5-year-old boy held hostage for six days in an underground bunker...