De Blasio was fined for misusing the NYPD as security during his failed 2020 presidential campaign.
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The NYC Conflicts of Interest Board fined the former mayor $155,000. He must also repay the city $319,794.20 for total travel costs from his campaign trips.
The board says he had New York cops accompany him on 31 out-of-state campaign trips.
De Blasio has filed a lawsuit to try to stop the decision.
His attorney says the ruling breaks with decades of NYPD policy and threatens the safety of public servants.
"Every Mayor faces threats, and all Mayors are entitled to protection - regardless of party or politics," his attorney, Andrew G. Celli, Jr., said. "That's why the NYPD is charged with determining what level of protection is necessary for such officials, when, and how. This case isn't about Bill de Blasio or any other Mayor or high official in particular. It's about the principle of protecting our leaders from harm - for the good of our City."
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The fine is "the largest fine in the history of the board."
"The Conflicts of Interest Board's conclusions regarding former Mayor Bill de Blasio's misuse of his security detail reaffirms DOI's investigative findings, and shows that public officials -- including the most senior -- will be held accountable when they violate the rules," said DOI Commissioner Jocelyn E. Strauber.
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