“The island’s credit unions represent the nest egg of nearly 1,000,000 Puerto Rican families (one of every four Puerto Ricans) that trust their livelihood and savings in these financial institutions,” said the credit unions’ primary regulator in a statement released last year, when the sector held Government Development Bank debt with a face value of slightly more than $500 million, according to regulatory records. The regulator’s spokesman did not answer messages Thursday.

Historically, the credit unions were required to invest only in very safe assets. But in 2009 their regulators made an exception, allowing them to buy and hold riskier bonds, as long as the bonds were issued by some branch of the Puerto Rican government. The change gave the Government Development Bank a new way to raise money, by selling its bonds to the credit unions.

As a result, those institutions wound up with outsize exposure to the bank, which itself was found insolvent last year by Puerto Rico’s Commissioner of Financial Institutions. Officials on the island have been seeking a way to protect the credit unions from the bank’s expected default, but it is not clear what the mechanism would be, or whether it could even be put in place in time.

A payment default by the Government Development Bank would add to the list of extreme measures that Puerto Rico has undertaken to stretch its dwindling cash while waiting for Congress to enact its rescue package. It has already removed assets from its workers’ compensation pool and public pension system to pay bills, taken cash from low-priority bonds to make payments due on high-priority bonds, and extended a highway privatization, giving up future toll revenue in exchange for upfront payments of $115 million.

On Monday, holders of roughly $9 billion of bonds issued by the island’s Electric Power Authority are scheduled to buy $111 million more in bonds as part of a complicated agreement to keep the power company liquid and preserve a consensual restructuring deal. Those investors must decide by Monday whether handing over $111 million is worth it, under the current confused and deteriorating circumstances. But Puerto Rico has warned that if the new money does not materialize it may sue the creditors.