Except as otherwise provided in section 261A.18, subsection 3, the authority may invest funds in direct obligations of the United States of America; obligations for which the timely payment of principal and interest is fully guaranteed by the United States of America; obligations of the federal intermediate credit banks, federal banks for cooperatives, federal land banks, federal home loan banks, federal national mortgage association, government national mortgage association and the student loan marketing association; certificates of deposit or time deposits constituting direct obligations of a bank as defined by chapter 524; and in withdrawable capital accounts or deposits of federal chartered savings associations which are insured by the federal deposit insurance corporation. However, investments may be made only in certificates of deposit or time deposits in banks which are insured by the federal deposit insurance corporation if then in existence. Securities authorized in this section may be purchased at the offering or market price at the time of the purchase. The securities purchased shall mature or be redeemable on dates prior to the time when, in the judgment of the authority, the funds invested will be required for expenditure. The judgment of the authority as to the time when funds will be required for expenditure or be redeemable is final.