Death threats have been sent to five journalists from Swiss tabloid newspaper Blick who have written about the growing violence in Swiss football stadia  the paper's publisher said Friday.

Four sports writers and a crime reporter were threatened by anonymous calls and messages overnight Thursday and found dead fish left in their letterboxes Friday morning, said a spokesman for the publishers Ringier.

Posters bearing the pictures of three of the five people, along with their private telephone numbers, were put up along the road used by the reporters' children to go to school.

"We have made a complaint and we hope that the culprits will be found," said Ringier's spokesperson.

Ringier would not reveal whether the journalists have been provided with any special security protection but conceded that those leaving the threats were "very well informed".

The five journalists had recently covered the story of a young Zurich football supporter who threw a firecracker in a match against Lazio in Rome on November 3rd injuring himself and around 30 others.

It was just one of a number of stories published by the tabloid on the worrying increase in violence at football matches.

But Ringier had "never had such violent threats" before, said the spokesman.