Federal regulators accused J.P. Morgan Chase & Co. and Royal Bank of Scotland Group PLC of duping five large credit unions into buying more than $3 billion in mortgage bonds that were "destined to perform poorly," and that quickly sank the credit unions.

The two civil lawsuits filed Monday in U.S. District Court in Kansas City, Kan., by the National Credit Union Administration are the most aggressive move yet by U.S. regulators to recover losses from Wall Street firms for alleged wrongdoing before and during the financial...