An IRS audit is really a civil matter where the general purpose is to uncover errors that the taxpayer may have made in filing a return.  In recognition of the fact that the tax law is extremely complicated, an audit will usually only result in an adjustment to the amount of tax owed and perhaps penalties for understating income.  A criminal investigation, on the other hand, may be handled by the Department of Justice and the IRS Criminal Investigation Division and is conducted with the aim of locating enough evidence to support an indictment and criminal conviction.  These investigations can take a substantial amount of time—in some cases, the investigation will last years before the authorities determine whether sufficient evidence exists to justify an indictment.