Wall Street investors like Blackstone have recently bought large amounts of homes, converting them into rental properties.

Now they are issuing bonds based on these properties, and housing activists argue the bonds put the entire housing market at risk. That's because the investors are driving up rental rates, and at some point the renters may not be able to afford to pay, the activists say, according to The Huffington Post.

"We are poised to experience another crisis if federal regulators fail to recognize and take corrective action to address red flags that are all too familiar," more than 75 housing and consumer associations wrote in a letter to federal bank and housing regulators

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The groups include the California Reinvestment Coalition and the National Consumer Law Center.

The letter seeks "immediate federal intervention" to assist qualified home borrowers who are now unable to garner affordable mortgage loans, as investors snap up almost half of all available homes with cash.

As for the bonds, Standard & Poor's declined to give them a triple-A rating last week, citing concerns about "the industry's operational infancy" and "historical performance" among others.

In a December report, Federal Reserve economists expressed worries about landlords taking on too much debt.

"Financial stability concerns may become more significant should debt financing become more prevalent or if the share of homes owned by investors in certain markets rises significantly further," the Fed economists wrote, according to The New York Times.

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