Ms Guerin, 36, one of Ireland's best- known journalists, was shot six times at close range by two gunmen on a motorcycle who ambushed her car at traffic lights in Clondalkin, a western suburb of Dublin.
It was the second time in 18 months she had been shot. The first attack took place in January last year at her north Dublin home when she was wounded in the leg. As in yesterday's shooting, the gunman wore a crash helmet.
Three months before that, warning shots were fired through her front window. Dublin's lowlife wanted her to stop publicising their business. Her response was to root deeper into their secrets and put them in print.
Last weekend Ms Guerin, crime correspondent with the Sunday Independent, wrote a shocking account of the life of one of Dublin's most notorious heroin dealers: Tony Felloni, known to gardai and media as "King Scum". She told how he introduced his own daughter, now an Aids victim, to the drug, and also tried to poison his wife, Anne, whom he repeatedly and savagely battered. Felloni was jailed last week for 20 years.
In the same issue she also wrote extensively on the IRA's killing of a garda detective in Adare, Co Limerick, earlier this month. She reported that an abortive raid on a postal delivery truck had been sanctioned by the Dublin-based operations director of the IRA's southern command.
Mother to a five-year-old son, she had been warned by friends and colleagues that she would again be a target because of her unflinching and minutely detailed coverage of the principal gang leaders in the city.
Despite widespread knowledge of her dangerous work and previous threats to her life, her death yesterday stunned Ireland - all other news items were dropped from bulletins. The Dail paid tribute with a minute's silence.
Irish government sources do not believe the IRA would have shot her. It is widely believed the order to kill her was given by one of the dozen senior organised crime figures she had followed assiduously over the past five years.
An accountant by training, Ms Guerin entered journalism late, at the age of 30, after working in business and public relations. Nevertheless, last December she received an international press freedom award from the Committee to Protect Journalists at a ceremony in New York.
Her research was minute. In covering frauds she went to South Africa and Nigeria to pursue key figures in her stories. She wrote at first for the Sunday Business Post and was for a time based in the Independent's Dublin office. She later moved to the Sunday Tribune and RTE television, before becoming the crime correspondent of the Sunday Independent.
She reported Dublin's pivotal position in drug trafficking in Ireland which has become a major problem for police and social services and will be a major focus when Ireland assumes the presidency of the European Union next week.
Drug pushers have become so pervasive in some sections of the capital and major cities that parents have formed vigilante gangs to keep them away. Last month, a vigilante gang beat a suspected pusher to death.
The ruthlessness of the attack and Ms Guerin's high-level connections mean the killing will inevitably prompt a wholesale review of organised crime policing in the Irish capital. Ms Guerin appeared frequently on Irish television and radio debates and as a commentator on crime.
Her employer for the last three years, Tony O'Reilly's Independent Newspapers, had offered her 24-hour protection after last year's shooting, which followed detailed reporting by her of the gang blamed for the largest cash robbery in the history of the Irish state six days earlier. A few months before she had been badly beaten when she went to question a suspected drugs wholesaler at his home outside Dublin. She was unfazed by the beating as she had been by previous death threats.
Aengus Fanning, editor of the Sunday Independent, summed up widespread feeling yesterday when he described her murder as "the ultimate attack on freedom of speech".
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