NEW YORK — New York City Mayor Bill de Blasio misused city resources by taking his police security detail with him on trips around the U.S. during his ill-fated presidential run, at a cost of around $320,000, and by letting his adult son use the detail as a free taxi service, investigators concluded in a report released Thursday.

The city's Department of Investigation, an independent agency that acts as a inspector general, also said a police van was improperly used to carry a futon when de Blasio's daughter, Chiara, moved from a Brooklyn apartment back to Gracie Mansion, the mayor's official residence, in August 2018. Members of the mayor's security team drove his son, Dante, around town and to and from Yale University, in New Haven, Connecticut.

Both de Blasio children were entitled to full-time protection but declined it as adults, the report said.

Dante de Blasio's frequent use of officers from the mayor's security detail for transportation did not arise from any security concern "but was instead driven solely by a combination of Dante's preferences and the availability of personnel who were otherwise assigned to the Mayor," the investigators concluded.

The department laid much of the blame for the misuse of the detail on the police department, saying it had no written procedures defining when and how the detail should be used.

"At the root of nearly all of these issues is the complete lack of any written policies or procedures at the NYPD for the operation of the mayoral security detail," the agency's report said. "Because it does not exist, neither the NYPD nor, as far as DOI is aware, any other City entity provided the Mayor, his family, or his representatives with any guidance – written or otherwise – concerning the appropriate or inappropriate uses of the security detail."

A spokesperson for the mayor, Danielle Filson, said the Department of Investigation report was "based on illegitimate assumptions and a naïve view of the complex security challenges facing elected officials today."

"Intelligence and security experts should decide how to keep the mayor and his family safe, not civilian investigators," she said.

The Department of Investigation said that either the mayor or his political campaign should reimburse the New York Police Department for the $319,794 it spent having de Blasio's security detail travel outside of New York City on 2019 presidential campaign trips.

A message seeking comment was sent to the police department.

De Blasio's term as mayor ends in December. The Democrat has said he is still interested in public service and is weighing options that could include a run for governor.