Extremists and hate groups aren't new. What is new is their use of social media and technology, allowing both the amplification of their vile messages and the rapid development of new networks, some of them global.

The resurgence of far-right political parties and leaders is adding to a toxic mix around the world, with increased scapegoating of immigrants and minorities. Reporting on such issues needs careful thought and clarity of purpose.

Three experienced journalists, who have reported on extremism in North America, Western Europe, and Argentina, share tips on how to investigate these groups.