TV highlights: NBC investigates the Boston Marathon bombing

Brian Williams hosts "108 Hours: Inside the Hunt for the Boston Marathon Bombers" (NBC at 8 p.m.), a breakdown of the time that elapsed between the bombing and capture of the second suspect. RETURNING SHOW: On the Season 6 opener of "Kitchen Nightmares" (Fox at 8), chef Gordon Ramsay pays a visit to the defiant owners of Amy's Baking Co. and the owner of a Denver pizzeria.