Islamic State militants captured in Iraq said the group has been planning attacks on subways in Paris and in the U.S., Iraqi Prime Minister Haidar al-Abadi said. The plotters are fighters from France and the U.S. who are in Iraq, he said.
http://mobile.bloomberg.com/news/2014-09-25/islamic-state-said-to-plot-subway-attacks-in-u-s-france.html
Unclear at this stage are when the authorities of both countries have been informed and if they have been allowed to question the suspects given they hold the citizenship of the targeted countries.