Whenever any officer refers or delivers a minor pursuant to Section 307.5, the agency to which the minor is referred shall immediately make such investigation as it deems necessary to determine what disposition of the referral or delivery should be made. If the referral agency does not initiate a service program on behalf of a minor referred to the agency within 20 calendar days, or initiate a service program on behalf of a minor delivered to the agency within 10 calendar days, that agency shall immediately notify the referring officer of that decision in writing. The referral agency shall retain a copy of that written notification for 30 days.