And sometimes it is more than just a sprinkling, as mutual funds look to increase their returns. The Oppenheimer Rochester Maryland Municipal Fund, for example, held more Puerto Rico bonds than Maryland bonds as of May 31: 49.7 percent compared with 45.3 percent. Maryland’s credit rating is AAA, so its low-risk bonds offer just a sliver of a reward. Puerto Rico bonds are riskier, so they yield more, and adding them to the mix increases the return.

The Oppenheimer Virginia fund is 40.2 percent Puerto Rico bonds, and its North Carolina bond fund has 35.3 percent. The holdings are disclosed on its website. A spokesman said Oppenheimer believed Puerto Rico had enough money to repay all of its debts.

As if that wasn’t enough, Puerto Rico made borrowing even more attractive. Its constitution contains an unusual clause that requires general-obligation bonds to be paid ahead of virtually any other government expense. And in case more reassurance was needed, the government created backstops, lockboxes and guarantee mechanisms for general-obligation and other types of debt, identifying specific revenue streams and promising them to certain groups of bondholders.

This practice is not at all unusual, especially in cases of deep distress, because local governments still have to borrow, even when they are broke.

Puerto Rico has been making pacts like that for years, locking up more and more of its resources to secure more and more bonds. That made them easier to sell, which in turn reduced borrowing costs at the outset. But over the long term, it left less and less money to provide essential government services.

“This is the main problem with the muni market,” said Matt Fabian, a partner at Municipal Market Analytics. “We don’t worry, generally, about how bond proceeds are spent. We worry about how bonds are repaid.”