The Boston Red Sox were banned from signing any international players for the next year and had five of their teenage prospects declared free agents on Friday after a Major League Baseball investigation showed they circumvented signing-bonus rules last year, a source familiar with the situation told Yahoo Sports.

Boston was limited last year to spending a maximum of $300,000 on international prospects after exceeding its spending limit the year before by spending $62 million on Cuban prospect Yoan Moncada. The Red Sox skirted the $300,000 threshold by packaging highly regarded prospects with lesser ones, paying both similarly and allowing the players’ agent to give the lion’s share of the money to the better prospect, according to the source.

The five players declared free agents are outfielders Albert Guaimaro and Simon Muzziotti, infielders Antonio Pinero and Eduardo Torrealba, and right-handed pitcher Cesar Gonzalez, according to the source. They are free to sign with any team starting Saturday, the start of the new international signing period, and the first $300,000 of their bonuses will not count against teams’ bonus-pool limits. The players, according to the source, will keep their original signing bonuses from Boston.

While the Red Sox still will be able to sign major league international free agents, the one-year ban is a significant blow to their development system. The Red Sox have thrived in Latin America, with third baseman Rafael Devers and right-hander Anderson Espinoza two of their best prospects and outfielder Manuel Margot and shortstop Javier Guerra the centerpieces of the trade that landed them closer Craig Kimbrel.

Guaimaro, 17, is considered the best of the prospects to be declared free agents. He is hitting .253/.305/.425 for the Red Sox’s Dominican Summer League team. Like Guaimaro, the 17-year-old Muzziotti (.317/.354/.383), Pineiro (.198/.258/.198) and Torrealba (.247/.318/.247) will be required to hire new agents before they sign, according to the source.

The sanctions, which Baseball America’s Ben Badler first suggested could be harsh, are a clear message from MLB: No matter how screwed up the international system may be – and most parties agree: it is – the penalties for breaking the rules will be severe.

The distribution of international amateur talent already has been part of the negotiations between MLB and the MLB Players Association on a new collective-bargaining agreement, multiple sources told Yahoo Sports. Whether that means an international draft – the desire of MLB – or a system that better distributes the talent remains an open question, sources said.