Standard & Poor’s downgraded three insurance companies, including Principal Financial , citing expected losses on commercial mortgages and commercial mortgage-backed securities, or CMBS.

Thursday’s downgrades come after an industry-wide review led the rating agency to focus on “several U.S. life companies with significant exposure to commercial mortgages and CMBS relative to total invested assets or potential losses relative to capital under stress scenarios.”

Also getting downgraded Thursday were mutual insurers NLV, which is part of National Life Insurance, and Pacific LifeCorp, which runs Pacific Mutual and Pacific Life. They and Principal are now rated triple-B, the second-lowest investment grade rating.

S&P affirmed its triple-A rating on the biggest mutual insurer, New York Life, and raised its outlook to stable from negative.* It also kept MetLife’s (MET) A-minus rating on watch.

Slumping commercial real estate values have been looming over the banking industry ever since the credit bubble collapsed in 2007. The Federal Deposit Insurance Corp.’s inspector general warned in 2008 that commercial real estate concentrations “have reached record levels that could create safety and soundness concerns in the event of a significant economic downturn.”

Just such a downturn ensued, and since then, more than 200 banks have failed — many of them smaller institutions that gorged on construction and land development loans during the housing boom.

S&P isn’t predicting any of the insurers mentioned will fail, but it said their balance sheets and their ratings could come under more stress.

“The downgrades announced today reflect our view that the affected groups are unlikely to be able to achieve capital adequacy necessary to support higher ratings in the near term,” the rating agency said.

Principal Financial, which is up 27% this year and has been trading within a dollar of its 52-week high, fell 1% in mid-morning trading Thursday, while MetLife stock was flat.

*Update 1:37 pm: Earlier I erroneously said S&P had cut its outlook on New York Life to negative from stable.