Real estate investment trusts have not typically been huge participants in the housing finance market, but it seems the tide is quickly turning.

According to a recent article in The Wall Street Journal, mortgage REITs have beefed up their mortgage-bond portfolios by nearly 28% year over year as of March, boosting their total collective investment to $308 billion.

Quoting data from Inside Mortgage Finance that analyzed 15 REITs, the WSJ calls this investment the largest stockpile in six years and says most of this rapid growth can be attributed to Annaly Capital Management and AGNC Investment.

While the REITs involved in home loans are small-time considering the overall market, the article says they have become an important source of capital as the Federal Reserve cuts down its bond portfolio.

And, if GSE reform is indeed underway as numerous developments indicate, REITs have a lot to gain from a new system that welcomes more private capital.

But there are those who call growing REIT involvement worrisome.

“Some analysts worry these vehicles are putting more of the mortgage market into the hands of leveraged firms with minimal oversight,” the article states. “Some of the more risky mortgage REITs went bust during the last financial crisis. The last boom time for this sector, in 2013, prompted calls for increased regulatory supervision, but they didn’t result in any new regulations.”

The WSJ says that REITs are making a comeback, and that those focused on the mortgage sector added $6.2 billion in equity to their investment portfolios last year, a number that is likely to be even bigger this year.

While REITs have produced mixed results for investors in recent years, according to the Journal, proponents say their structure makes them ideal to become the mortgage industry’s backbone as they can raise and deploy funds quickly and can manage risks carefully.

Some firms have been buying mortgages that have traditionally been in Freddie Mae and Freddie Mac’s territory, and the article says this development could pick up speed if the government shrinks the role of these GSEs.

“REITs are big buyers of securities sold by Fannie and Freddie that transfer default risk associated with the mortgages they back,” the article states. “In a sign that traditional players are making room for REITs, Fannie and Freddie recently tweaked these investment vehicles to make them easier for REITs to buy.”