I don't see any such claim, but the idea that prosecutors correctly identify the perpetrator in 98% of cases is obviously pure fantasy.
In 98% of cases they bring for prosecution - they know the job, they know what works, and if they don't think they can follow through, they often just drop the case entirely, so it doesn't count.
In 98% of cases they bring for prosecution - they know the job, they know what works, and if they don't think they can follow through, they often just drop the case entirely, so it doesn't count.