Federal authorities investigating whether President Trump’s former personal lawyer and fixer, Michael D. Cohen, committed bank and tax fraud have zeroed in on well over $20 million in loans obtained by taxi businesses that he and his family own, according to people familiar with the matter.

Investigators are also examining whether Mr. Cohen violated campaign finance or other laws by helping to arrange financial deals to secure the silence of women who said they had affairs with Mr. Trump. The inquiry has entered the final stage and prosecutors are considering filing charges by the end of August, two of the people said.

Any criminal charges against Mr. Cohen would deal a significant blow to the president. Mr. Cohen, 52, worked for the president’s company, the Trump Organization, for more than a decade. He was one of Mr. Trump’s most loyal and visible aides and called himself the president’s personal lawyer after Mr. Trump took office.

The bank loans under scrutiny, the total of which has not been previously reported, came from two financial institutions in the New York region that have catered to the taxi industry, Sterling National Bank and the Melrose Credit Union, according to business records and people with knowledge of the matter, including a banker who reviewed the transactions.