Far from deferring to state or local officials, FEMA asserted its authority to make things worse, said Aaron Broussard, the president of Jefferson Parish, south of New Orleans. When Wal-Mart sent three trailer trucks loaded with water, FEMA officials turned them away, he said. Agency workers prevented the Coast Guard from delivering 1,000 gallons of diesel fuel, and on Saturday they cut the parish's emergency communications line, leading the sheriff to restore it and post armed guards to protect it from FEMA, Mr. Broussard said.