More than three dozen letters sent to D.C. schools containing a suspicious white powder and references to al Qaeda are being analyzed at the FBI’s Virginia lab before officials ship them to Dallas, where a larger investigation is unfolding. In Quantico, Va., agents are conducting additional tests to identify the powder, which is apparently nonhazardous, that caused several District schools to evacuate or lock down on Thursday and Friday. Forensic analysts will review the 39 letters, postmarked from Dallas, before sending them to Texas.