WASHINGTON (Reuters) - Former FBI chief James Comey said on Thursday that he found the shifting explanations for why President Donald Trump had fired him both confusing and concerning. “When I was appointed FBI director in 2013, I understood that I served at the pleasure of the president,” Comey told the Senate Intelligence Committee. “On May the ninth, when I learned that I was fired, for that reason, I immediately came home as a private citizen,” he said. “But then the explanations, the shifting explanations confused me and increasingly concerned me.”     