The former Syrian military crime-scene investigator who helped document years of suspected torture and killing under President Bashar al-Assad is scheduled to tell his story to U.S. lawmakers on Thursday as part of a concerted effort to pressure the U.S. government to take more decisive action in the conflict.

Known by the pseudonym Caesar, the middle-aged Syrian military defector arrived in Washington on Monday for a weeklong series of meetings with lawmakers and members of the Obama administration who have examined thousands of graphic photographs he helped smuggle out of Syria. Caesar’s work, based on interviews with war-crimes investigators, defectors, and opposition leaders working with Caesar, was the subject of an in-depth WSJ story.