A female union official has been shot dead at South Africa's Marikana mine, where 34 striking miners were killed by police a year ago, her colleagues say.The shop steward from the National Union of Mineworkers (NUM) was killed on Monday morning.The NUM described the death as "cold-blooded murder," and said it was "outraged".Tension between rival unions at the Lonmin-owned platinum mine has led to several shootings over the past year.Local NUM spokesperson Mqashi Sthethi told the BBC that the woman, who was in her 50s, was shot outside her home at 10:30 local time (08:30 GMT).'Assassinations'It is not clear whether Monday's killing is connected to the conflict between the NUM and the Association of Mineworkers and Construction Union, which was formed last year.The NUM said it was "disappointed" at the lack of progress in arresting suspects for what it termed the "assassinations" in Marikana.The victim's niece told reporters that everybody was scared."If you start talking, start doing anything, you don't know what might happen to you," Thandi Mateyisa said.A spokesperson for Lonmin, Sue Vey, told Reuters that the South African police were investigating the death.The shootings of 16 August 2012 were the most deadly police action in South Africa since the end of apartheid.The BBC's James Copnall in Johannesburg says last year's deaths dented foreign investor confidence in South Africa, and any hint of further trouble at Marikana is scrutinised closely at home and abroad.A Commission of Enquiry continues to investigate the killings.