The Department of Justice and the New York attorney general soon opened investigations into the industry. The city arrested a debt collector, waived $10 million in fees owed by medallion owners and strengthened regulations.

The taxi titans expanded their operations to Chicago …

Symon Garber , a New York fleet owner, along with a group of partners, began buying medallions in Chicago and lending to other buyers. They eventually bought 800 of the city’s 7,000 medallions.

Michael Levine, a legend in New York’s taxi industry, bought more than 500 medallions in Chicago. Mr. Levine also was involved in a company that provided at least 750 loans to medallion owners.

At least 40 other New Yorkers bought Chicago medallions, including Michael Cohen, President Trump’s former lawyer, records show.

… and to Boston, Philadelphia and elsewhere.

Then some of the same people who roiled New York’s industry expanded their operations. Medallion prices soared to $700,000 in Boston, $550,000 in Philadelphia, $400,000 in Miami and $250,000 in San Francisco.

But in Chicago, New Yorkers eventually bought almost half of that city’s medallions, records showed. The average cost of a medallion there — less than $50,000 in 2006 — rose to nearly $400,000 before prices began plummeting in 2013.

Today, a Chicago taxi medallion is worth $30,000 or less.

“In retrospect, it should’ve set off alarm bells” that New Yorkers were entering Chicago’s market,” said Michael Negron, who was a policy adviser to Rahm Emanuel, a former Chicago mayor. “Outside investors were coming in to upend the industry, and everybody kind of missed it.”