The attacks in Paris last week were a deadly demonstration that the Islamic State, once a group of militants focused on seizing territory in Iraq and Syria, has broadened its focus to attack the West. The electronic files seized in the Pentagon investigation tell the story of the group’s rise, as seen through the eyes of Centcom, which oversees military operations across the Middle East.

The exact content of those documents is unclear and may not become public because so much of the information is classified. But military officials have told Congress that some of those emails and documents may have been deleted before they had to be turned over to investigators, according to a senior congressional official, who requested anonymity to speak about the ongoing inquiry. Current and former officials have separately made similar claims, on condition of anonymity, to The New York Times. Although lawmakers are demanding answers about those claims, it is not clear that the inspector general has been able to verify them. A spokeswoman for the inspector general declined to comment.

Staff members at the House Permanent Select Committee on Intelligence are also poring over years of Centcom intelligence reports and comparing them to assessments from the Central Intelligence Agency, the Defense Intelligence Agency and others. The committee is not just examining reports about Iraq, Syria and the Islamic State, but also about Afghanistan and other areas under Centcom’s purview. The insurrection inside Centcom is an important chapter in the story of how the United States responded to the growing threat from the Islamic State. This past summer, a group of Centcom analysts took concerns about their superiors to the inspector general, saying they had evidence that senior officials had changed intelligence assessments to overstate the progress of American airstrikes against the Islamic State, also known as ISIS.