Indonesian investigators were sifting through the rubble at Jakarta's stock exchange complex Tuesday as they looked for clues on what caused a walkway collapse that left scores injured, including dozens of students.A mezzanine floor at the tower in the sprawling city's business district collapsed shortly before lunchtime Monday, injuring a total of 73 people, police said, adding that there were no deaths.Dramatic CCTV footage showed a group of about 40 visiting students on a balcony