Mortgage investors are borrowing from government-chartered Federal Home Loan Banks, raising concerns from their regulator that the trend creates unacceptable risks.

Three real estate investment trusts -- Annaly Capital Management Inc., Invesco Mortgage Capital Inc. and Two Harbors Investment Corp. -- set up insurance units that allowed them to gain access to an FHLB in the past seven months. Commercial real-estate lender Ladder Capital Corp. joined the network of 12 regional cooperatives in 2012. Two Harbors and Ladder had borrowed a total of about $1.5 billion through March.

Lightly regulated investment firms and lenders that lack customer deposits are flocking to the FHLBs for dependable funding, often with better terms than traditional banks or debt markets. Their memberships are drawing scrutiny from the home loan bank overseer, the Federal Housing Finance Agency, because they may affect the safety of a system that operates with perceived taxpayer backing. FHFA Director Melvin Watt said last month he saw “possible issues” with the insurers’ access.

“Just because it’s legal doesn’t mean it’s not risky,” said Karen Shaw Petrou, managing partner of Washington-based research firm Federal Financial Analytics Inc. The companies “are technically eligible borrowers because of carefully constructed windows. These are not traditional banks. They’re very different.”

Shadow Banks

Mortgage REITs, which aren’t required to pay taxes if they pay out most of their earnings in dividends, invest in everything from government-backed securities to residential loans too large for federal programs and commercial property debt. The 44 members of a Bloomberg index of publicly traded mortgage REITs, some of which are run by money management firms, own almost $500 billion of assets. Ladder Capital, which isn’t a REIT or bank, makes commercial mortgages to package into bonds.

The firms are part of the shadow banking system because they lack the regulatory oversight and emergency funding tools of traditional banks. They’re joining FHLBs through so-called captive insurers, which are units that mostly serve needs of the parent companies.

“We want to and plan to address the issue of captives,” said Peter E. Garuccio, an FHFA spokesman. “We are looking at our toolkit of what we should do in terms of the next steps.”

The two home loan banks lending to these companies said that the risks are limited by practices that have kept FHLBs from ever suffering losses on their loans. Expanding funding options for the firms may also be a boon for property buyers and real-estate markets that the system is meant to bolster.

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