Federation Internationale de Football Association have been aggressive in their pursuit of youth transfer rule breaches in recent years, with Atletico Madrid's current transfer ban a result of the club signing underage players outside of the regulations.
Atletico Madrid are now under a transfer embargo for breaching similar rules about the signing of minors and their appeal was upheld by the Court of Arbitration of Sport (CAS).
"We can not provide any details concerning the matters under investigation".
The only other exception is for transfers within either the European Union or European Economic Area (EEA), where the minimum age is 16. They appealed against the decision but it was upheld by the court of arbitration for sport.
Federation Internationale de Football Association banned the London club from signing players for two transfer windows in September 2009 due to Gael Kakuta's 2007 move from Lens, a decision which Chelsea successfully appealed at the Court of Arbitration for Sport (CAS).
Real Madrid have also been punished in the recent past and had their ban cut from two transfer windows to one.
And in 2016, the Premier League champions were also investigated over the transfer of Burkina Faso global Bertrand Traore who was signed after his 18th birthday but had appeared in an under-18 match in 2011 while still 16. Traoré signed for Chelsea after his 18th birthday.
Liverpool and Manchester City have also been given a slap on the wrist by the Premier League for their conduct regarding the signing of academy players. Both of them were found to have tapped up young players. They were fined £100,000.