WASHINGTON — A group of intelligence analysts have provided investigators with documents they say show that senior military officers manipulated the conclusions of reports on the war against the Islamic State, according to several government officials, as lawmakers from both parties voiced growing anger that they may have received a distorted picture about the military campaign’s progress. The Pentagon’s inspector general, who is examining the claims, is focusing on senior intelligence officials who supervise dozens of military and civilian analysts at United States Central Command, or Centcom, which oversees American military operations against the Islamic State in Iraq and Syria....