Who are the main dissident republican groups?

Northern Ireland's First Minister, Peter Robinson, has issued a public appeal over the murder of Constable Ronan Kerr, urging anyone with information to come forward.

As yet, no group has claimed responsibility for the car bomb attack in Omagh, but it has been been blamed on dissident republicans opposed to the peace process.

Our Northern Ireland political editor Mark Devenport considers those most likely to be under scrutiny now.