The U.S. and its European allies must work together to stop radicalized Westerners who travel to terrorist training camps and return home to wage attacks, counterterrorism experts said in a new study.

The warning came as Western officials this week investigate an active terror plot they said was aimed at carrying out Mumbai-style shooting rampages or other attacks in Britain, France and Germany. A Pakistani intelligence official has said a number of Germans and British militants were involved.