The US Justice Department has opened a criminal investigation involving the leaked Panama Papers on offshore shell companies, according to an official letter to the group that brought the records to light.

The letter from the office of Manhattan US Attorney Preet Bharara, obtained by AFP Wednesday, asks to discuss the investigation with the International Consortium of Investigative Journalists, which worked with media outlets to reveal the documents.

The letter, dated April 3 and obtained from the ICIJ, said Bharara's office "has opened a criminal investigation regarding matters to which the Panama Papers are relevant."

His office said it "would greatly appreciate the opportunity to speak as soon as possible" with someone from ICIJ on the matter.

The letter gave no information on the subject of the investigation, and was not a subpoena that would force the ICIJ to turn over any of the 11.5 million confidential documents leaked from Panama law firm Mossack Fonseca.

Access to the documents remains limited to a handful of international media organizations.

Reports from the ICIJ and the media groups based on the Panama Papers link a number of international leaders, including Russian President Vladimir Putin, British Prime Minister David Cameron, and leaders and top officials in Ukraine, China and Argentina, to offshore shell companies that can be used to hide wealth and launder money.

Iceland's Prime Minister Sigmundur David Gunnlaugsson was also forced to resign after the documents showed he and his wife made use of an offshore company.