Del. Code tit. 30, § 364

For the purpose of ascertaining the correctness of any return or for the purpose of making an estimate of the taxable income of any taxable, the Secretary of Finance may examine or cause to be examined, by any agent or designated representative, any books, papers, records or memoranda bearing upon the matters required to be included in a return and may by summons require the attendance of the taxable or of any other person having knowledge in the premises and may take testimony and require proof material for the investigation, with power to administer oaths to such person or persons. All banks, trust companies and brokers, when required by the Secretary, shall allow a representative of the Department of Finance to verify all accounts and records pertaining to the income of any taxable.