[Leading Private Investigation Firm | Special Investigations Unit -NSIU.com National SIU is a private investigation firm offering an array of services to corporate and private clientèle. We produce effective evidence for our clients across the United States and, in some cases, around the world. Our firm is split into two divisions to meet the highly specific needs of our clients: a Special Investigations Unit (SIU) to combat insurance fraud, and a Private Services Division for private and corporate clients. Our work has been featured on ABC’s 20/20.]