Housing and consumer activists warn that Wall Street is about to crash the housing market -- again.

The activists said they are particularly concerned about the growing number of companies looking to issue bonds backed by rental properties -- bonds that a coalition of groups described as "eerily like" those mortgage-backed securities that helped fuel the last housing bubble.

"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 groups wrote in a letter Tuesday to federal bank and housing regulators.

The 2008 housing crisis happened because banks were willing to give even risky borrowers a mortgage, driving home prices to unsustainable peaks. Those mortgages got sold into bonds that defaulted once homebuyers stopped making their monthly payments.

This time, gun-shy bankers are hard-pressed to give anyone but the most stellar borrowers a mortgage, said the groups, which include California Reinvestment Coalition and the National Consumer Law Center. Yet, home prices are rising again.

That's because Wall Street investors with deep pockets and the ability to pay cash for homes are muscling out ordinary buyers in places hard-hit by the housing crisis, like Phoenix and Atlanta. Once these wealthy investors have bought the homes, they flip them into rentals -- often covering up large issues like plumbing and mold with cosmetic fixes.

The letter demands "immediate federal intervention" to rebalance the housing market in favor of qualified borrowers who currently can't get affordable mortgage loans, and away from Wall Street and other cash investors who in some markets are buying nearly half of all available houses.

The activists cited a previous HuffPost report that described how large investment companies sometimes prove to be lousy landlords.

The letter warns a housing market dominated by all-cash buyers may keep lending standards high, allowing big companies to further tighten their grip. The letter is addressed to the Federal Reserve, the Department of Housing and Urban Development, the Office of the Comptroller of the Currency, the Federal Housing Finance Agency and the Consumer Financial Protection Bureau.

The origins of the threats described by the groups are complex, and have their roots in the 2008 financial crisis. After the crash, the government bailed out two of the biggest players in the home loan market, Fannie Mae and Freddie Mac, and committed to helping them return to profitability. That vow came at the expense of ordinary homeowners, advocates argued, because Fannie and Freddie will only back loans for buyers with the highest credit.

In the years since, Wall Street-backed companies have swooped into markets like Phoenix, Atlanta and Memphis, buying houses that had plunged in value and then renting them out. The biggest company operating with this business model is Dallas-based Invitation Homes, backed by Blackstone, the biggest private equity firm in the world. Other prominent investment companies are American Homes 4 Rent, based in Agoura Hills, Calif., and Colony American Homes, based in Scottsdale, Ariz.

Some housing experts credit these buyers with spurring a housing price recovery. But with that recovery has come fears of a new bubble.

Blackstone announced in October it would sell the first bonds backed by income from single-family rental homes, valued at $479 million, according to The Wall Street Journal.

In November, Colony, which owns and manages more than 15,000 rental homes, announced it was exploring doing the same. American Homes 4 Rent is working with Goldman Sachs to set up a bond offering of its own, Bloomberg reported in January.

The rental income for these bonds comes from the more than 200,000 homes these companies have flipped into rentals. The market for the new bonds is predicted to top $70 billion a year by 2016, according to the Center for American Progress, a left-leaning think tank.

Critics say the bonds are risky. Rep. Mark Takano (D-Calif.) has called for congressional hearings into what would happen if a future economic downturn leaves tenants unable to pay their rent and causes vacancies to rise.

In Takano's district, in Riverside, Calif., nearly one-third of tenants are spending more than half their income on rent, according to a recent report his office commissioned. Takano said stricter lending standards, and stepped-up competition from institutional investors, have forced more people into rental homes, driving up home prices.

The credit ratings agency Standard & Poor's announced last week that the rental-backed bonds don’t yet merit the top triple-A credit rating. S&P said it was concerned about the risk associated with managing so many properties spread widely across the country.

Kevin Stein, associate director of the California Reinvestment Coalition, said the conversion of rental income into financial products will fuel more home buying, and even more rental conversions. He said he hopes the concerns raised in the letter prompt regulators to take action to prevent the kind of risk-taking that set off the last housing bubble. "We are hoping this isn't news to them," he said.

In order to keep paying investors, the rental companies must ensure vacancy rates remain low. This means keeping homes in good shape, so people want to live there.

Last fall, HuffPost uncovered that renters were moving into homes, then discovering major plumbing and other household repair flaws that their Wall Street landlords wouldn’t fix.

Emily Carlson, 34, lives in a house in Atlanta that her family rented from Colony American Homes last year. First, the air conditioning broke, leaving her three children -- all younger than 6 -- to swelter in the July heat. It took 2 1/2 weeks and repeated visits from repairmen before Colony would approve a fix, Carlson said.

The home, which her family rents for $2,375 a month, smells of human waste that Carlson said she believes is caused by faulty plumbing. Instead of properly removing mold on the walls, the company apparently just painted over it -- and now it has reemerged.

Colony declined to comment.

In December, Carlson’s family squeezed into one room after the heat broke. She said she feels unsafe at the house when her husband is working late nights because outside doors don't reliably lock.

Carlson said she is afraid to break the lease, because it would damage her credit. She and her family want to buy a home, but they can’t beat the stiff competition posed by her landlord and other investment companies. In Atlanta, institutional buyers, like Colony, accounted for one in four sales in December, according to the housing market firm RealtyTrac.

These buyers pay cash, whereas Carlson, like most ordinary people, would have to take out a mortgage. Yet for all her landlord's financial might, it can't seem to find the funds to fix her front door.