WEST VALLEY CITY, Utah — It began with bullets and bloodshed one November afternoon. A 21-year-old woman was dead. Two undercover officers had opened fire on her car. The police began asking the usual questions about what had happened, and why.

Their investigation cracked open what one prosecutor called a “Pandora’s box of problems” here in Utah’s second-largest city, where Mormon pioneers once raised milk cows and sugar beets. There have been accusations of stolen drugs and missing money, abuses of police power and a cloud of corruption that defies Utah’s reputation for sunny optimism.

Over the past few months, accusations of bad police work in the narcotics squad of the West Valley City Police Department have engulfed the town and sent shock waves through Utah’s justice system. Prosecutors have tossed out 125 criminal cases. Dozens of convictions may have to be re-examined. The F.B.I. is investigating the Police Department and several officers.

Officials in Utah say they have never seen anything like it.

“Chaos,” said Sim Gill, the district attorney for Salt Lake County.