The department superintendent at the time, Benjamin M. Lawsky, a former aide to Gov. Andrew M. Cuomo, said he did not, as a rule, discuss his tenure at the department.
In an emailed statement, the department denied it struggled after the merger and said it took action to stop the collapse of the medallion market. A department spokesman provided a long list of warnings, suggestions and guidelines that it said examiners had issued to lenders. He said that starting in 2012, the department downgraded some of its own internal ratings of the lenders.
The list did not include any instances of the department formally penalizing a medallion lender, or making any public statement about the industry before it collapsed.
Between 2010 and 2014, as officials at every level of government failed to rein in the risky lending practices, records show that roughly 1,500 people bought taxi medallions. Overall, including refinancings of old loans and extensions required by banks, medallion owners signed at least 10,000 loans in that time.
Several regulators who tried to raise alarms said they believed the government stood aside because of the industry’s connections.
Many pointed to one company — Medallion Financial, run by the Murstein family. Former Gov. Mario M. Cuomo, the current governor’s father, was a paid member of its board from 1996 until he died in 2015.
Others noted that Mr. de Blasio has long been close to the industry. When he ran for mayor in 2013, an industry lobbyist, Michael Woloz, was a top fund-raiser, records show. And Evgeny Freidman, a major fleet owner who has admitted to artificially inflating medallion prices, has said he is close to the mayor.