AHMEDABAD, India — The police stood by as Hindu mobs slaughtered nearly 1,000 people, mostly Muslims, in massacres that evidence suggests were an election-year ploy by state officials to garner votes. Mothers were skewered, children set afire and fathers hacked to pieces.

That was 10 years ago. A decade later, the riots in Gujarat State may be remembered less for the horrors they unleashed, however, than that such sectarian carnage, which once struck India as often as a heavy monsoon, has not been repeated since. There are many reasons for this astonishing quiescence, but technology has played a crucial role. The killers made cellphone calls, and records of those calls became evidence.

After years of dithering, India’s creaky justice system lurched into action. Hundreds of rioters have been convicted, and more cases are pending. On Saturday, a judge trying 61 defendants — including a former state education minister — delayed issuing verdicts until Aug. 29 in a case that involves about 94 deaths. A total of 327 people testified, but the crucial evidence, again, was the phone records contradicting claims by some of the accused that they were nowhere near the scene of the crimes....