The DOJ’s authority to engage in International Enforcement Cooperation is inherent in its ability to act in furtherance of their mandates.  DOJ has the discretion to cooperate, including when it furthers its enforcement interests.  See https://www.justice.gov/atr/internationalguidelines/download.  The IAEAA authorizes the DOJ and FTC to enter into antitrust-specific mutual assistance agreements with foreign authorities that allow the FTC and DOJ to share evidence relating to antitrust violations already in their possession and provide investigatory assistance in obtaining evidence, subject to certain limitations. See 15 U.S. Code Chapter 88 – International Antitrust Enforcement Assistance Act (IAEAA), https://uscode.house.gov/view.xhtml?path=/prelim@title15/chapter88&edition=prelim.