DETROIT — Prosecutors dropped all criminal charges Thursday against eight people in the Flint water crisis and pledged to start from scratch the investigation into one of the worst man-made environmental disasters in U.S. history.

The stunning decision came more than three years — and millions of dollars — after authorities began examining the roots of the scandal that left Flint’s water system tainted with lead. Michigan Solicitor General Fadwa Hammoud, who took control of the investigation in January after the election of a new attorney general, said “all available evidence was not pursued” by the previous team of prosecutors.