WASHINGTON — While Hurricane Florence barreled through the Carolinas, a different type of storm was brewing within the federal disaster relief agency tasked with responding to the fallout. The fate of Brock Long, the head of the Federal Emergency Management Agency, was cast in doubt with revelations that he was being investigated by Homeland Security’s internal watchdog and a congressional committee for the possible misuse of government vehicles. The watchdog, the inspector general’s office, has turned the review over to federal prosecutors to determine whether any criminal charges are warranted, according to a person familiar with the probe.