Puerto Rican municipal bonds have become one among the most widely traded in the world in recent months, and while most investors were unloading them in September, retail investors were still net buyers and crossover investors took advantage of the newly attractive prices. Selling subsided toward the end of the month, but the decision of state securities regulators to investigate the sale of Puerto Rican debt to Massachusetts residents could trigger further sell-offs, according to Citi analysts Mikhail Foux and Vikram Rai. Heavy trades of Puerto Rican municipal credit “Over the last two months Puerto Rico has become one of…