In 2011, a 16-year-old confessed to being the getaway driver in a fatal shooting of a Los Angeles man, leading prosecutors to charge the teen and three others with murder.

But defense lawyers found convenience-store video showing the suspects were nearly four miles from the murder scene a minute before the man was shot. The lawyers argued that a detective had coerced the teen, who was drunk, into confessing to a crime he didn't commit. Prosecutors last December dropped the charges—and the possibility the four would spend life...