The number of nonbank firms that service mortgages in the United States is growing, but the federal agency charged with protecting borrowers has no single list of the companies, even though they can pose major risks to consumers, according to a report released Monday.

In its report, the Government Accountability Office found the Consumer Financial Protection Bureau, which was created in response to the housing and financial crisis, did not have "comprehensive data on the identity and number of nonbank mortgage servicers in the market."

The GAO is an independent watchdog that regularly audits federal programs and agencies.

While there are benefits to having nonbanks such as Quicken Loans service mortgages, there are also risks that often fall between the regulatory cracks, the GAO found.

"Issues related to aggressive growth and insufficient infrastructure have resulted in harm to consumers, have exposed counterparties to operational and reputational risks and ... complicated servicing transfers between institutions," the GAO found.

During some transfers of mortgages, firms have lost documents or not credited borrowers' payments on loans, the GAO found.

"Some borrowers lost contact with their servicers, and their new servicers did not always receive or adhere to borrowers’ existing loss mitigation agreements with the previous servicer," the GAO found. "In some cases, these types of transfer errors may have resulted in some borrowers improperly losing their homes to foreclosure."

The GAO conducted the review at the request of two Democrats who have been vocal about tightening Wall Street regulation after the financial crisis: Rep. Elijah Cummings of Maryland and Sen. Elizabeth Warren of Massachusetts. Both responded to the findings by calling for CFPB to collect data and strengthen oversight of servicers.

"Collecting information on and regulating nonbank mortgage services to protect consumers is well within CFPB's statutory authority and core mission," they wrote to the bureau's head, Richard Cordray. "We hope that you take actions to do so as rapidly as possible."

The GAO found nonbank servicers still did not command as great a market share as banks but that their influence was growing.

In the second quarter of 2015, Wells Fargo, the largest U.S. mortgage servicer, commanded 17.1 percent of the market, compared with 18 percent in the first quarter of 2012, the report said.

Nationstar Mortgage, LLC, which was not listed as a top 10 servicer in 2012, ranked fourth in 2015. Its 4.1 percent market share was greater than that of Citi or PNC Mortgage.