His brother Sean died in December 1975, when a landmine he was preparing for an attack on the British Army exploded prematurely. His other brother Peter served 14 years in prison for Provisional Irish Republican Army (IRA) offences. His brother Michael went on trial in Vilnius, Lithuania, accused of trying to purchase arms and explosives for the Real IRA, was found guilty and sentenced to twelve years in jail. Liam Campbell first came to the attention of the British and Irish security forces in the early 1980s and became the subject of an exclusion order in 1983 barring him from entering Northern Ireland.
Campbell was named by the BBC as one of the perpetrators of the 1998 Omagh bombing in Panorama documentary.  He never faced criminal charges but was found liable for the bombing in a civil trial, along with Michael McKevitt, Colm Murphy and Seamus Daly.  The case, brought by relatives of the 29 victims, resulted in an award of combined damages of over £1.5m.  In May 2004 he was convicted by a court in the Republic of Ireland for membership of the Real IRA and was sentenced to eight years imprisonment.
- "Vilnius 'Real IRA' trial to open". BBC News. 2009-08-18. Retrieved 2009-08-18.
- "Dáil Éireann - Volume 345 - 25 October, 1983". Retrieved 2007-02-12.
- Hopkins, Nick (2002-07-27). "The five suspects families want to bring to court". London: The Guardian. Retrieved 2007-02-12.
- The men sued by the Omagh families, BBC News, Monday, 8 June 2009.
- "Four found liable for Omagh bomb". RTÉ News. 2009-06-08. Retrieved 2009-06-08.
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