Lenders did not respond to requests for comment. Previously, they denied wrongdoing, saying regulators had approved all of their practices and some borrowers had made poor decisions and assumed too much debt. Lenders blamed the crisis on the city for allowing ride-hailing companies like Uber and Lyft to enter without regulation, which they said led medallion values to plummet.

Mr. de Blasio said the city’s investigation will focus on the brokers who arranged the loans for drivers and sometimes lent money themselves.

“The 45-day review will identify and penalize brokers who have taken advantage of buyers and misled city authorities,” the mayor said in a statement. “The review will set down strict new rules that prevent broker practices that hurt hard-working drivers.”

Four of the city’s biggest taxi brokers did not respond to requests for comment.

Bhairavi Desai, founder of the Taxi Workers Alliance, which represents drivers and independent owners, said the city should not get to investigate the business practices because it was complicit in many of them.

The government has already closed or merged all of the nonprofit credit unions that were involved in the industry, saying they participated in “unsafe and unsound banking practices.” At least one credit union leader, Alan Kaufman, the former chief executive of Melrose Credit Union, a major medallion lender, is facing civil charges.

The other lenders in the industry include Medallion Financial, a specialty finance company; some major banks, including Capital One and Signature Bank; and several loosely regulated taxi fleet owners and brokers who entered the lending business.

At City Hall, officials said Monday they were focused on how to help the roughly 4,000 drivers who bought medallions during the bubble, as well as thousands of longtime owners who were encouraged to refinance their loans to take out more money during that period.