The idea of the PSI is generally credited to John R. Bolton, former US Under-Secretary of State for Arms Control and International Security and former United States Ambassador to the United Nations, after 15 Scud missiles found on board an unflagged North Korean freighter, the "So Sen", heading towards Yemen had to be released when it turned out that international law did not allow them to be confiscated. Given this apparent gap in international law, several months later US President Bush announced the initiative with his counterpart, Polish President Aleksander Kwaśniewski at Wawel Castle in Kraków on May 31, 2003.