Democrat lawmakers are under scrutiny as the Federal Bureau of Investigation (FBI) begins an investigation into allegations of illegal commands shown in a contentious video. The inquiry is being led by the bureau’s counterterrorism division, according to Senator Elissa Slotkin of Michigan. Slotkin, who previously served as a C.I.A. analyst, organized the video and confirmed the FBI’s involvement. The probe follows a separate Pentagon investigation into Senator Mark Kelly for his participation in the video.