Who is Fox News commentator Steven Emerson?

Fox News commentator Steven Emerson has apologised after claiming that Birmingham is "a totally Muslim city where non-Muslims simply don't go in".

So, who is the man who describes himself as 'one of the leading authorities on Islamic extremist networks, financing and operations'?

• High-profile journalist: Emerson has been an investigative correspondent at CNN, a senior editor for U.S. News and World Report and a freelance writer for The New Republic.

• Author of books on terrorism: Emerson is the author or co-author of six books on terrorism and U.S. national security. His titles include: 'Secret Warriors: Inside the Covert Military Operations of the Reagan Era' and 'American Jihad: The Terrorists Living Among Us'.

• A regular in Congress: Emerson has testified numerous times before committees of both houses of Congress on terrorist funding, and the operational structures of groups including al-Qaeda, Hamas, Hezbollah, and Islamic Jihad.

• Documentary maker: In 1994, Emerson made a documentary called 'Terrorists Among Us: Jihad in America' for PBS. In it, he tried to lay bare the workings of Islamist groups which were operating in the USA. In the film, Emerson stood in front of the Twin Towers and said: "The survivors of the explosion at the World Trade Centre in 1993 are still suffering from the trauma, but as far as everyone else is concerned, all this was a spectacular news event that is over. Is it indeed over? The answer is: apparently not. A network of Muslim extremists is committed to a jihad against America."

• Controversial figure: This is not the first time that Emerson has courted controversy. A reporter for The Nation accused him of "creating mass hysteria against American Arabs", while a liberal American think-tank called him a "misinformation expert".