CEDAR RAPIDS — Selling liquor, lottery tickets and tobacco, the Iowa Smoke Shop keeps a watchful eye on the goings on inside and outside the southwest Cedar Rapids store.When its parking lot became the scene of a double murder early May 18, its surveillance cameras were rolling, capturing high-quality video that proved a crucial — and yet an increasingly common — tool for investigators.“I’d guess at least 70 to 80 percent of the cases we work on have some sort of video component, and that’s probably a low estimate,” said Brent Long, a captain in the Investigation Division of the Cedar Rapids Police Department.Surveillance video not only enables investigators to see what happened during an incident, but also can lead to identifying suspects or witnesses connected to the case.