THE comments started appearing on American media websites in May. A wealthy New York banker had allegedly stolen Tommii Cosgrove’s wife and Cosgrove was not going to let the matter rest. “The story doesn’t end until I say so,” the cuckolded husband wrote in one inflammatory rant.

Three months later, the quiet resignation of a prominent financier from the merchant banking arm of Credit Suisse has sent shudders through Wall Street and sparked an angry debate about internet users who pursue private grudges in public forums.

The banking career of Steven Rattner, a Credit Suisse managing director, has been wrecked by an internet vendetta that has also caused unwanted headaches for a different Steven Rattner, the billionaire founder of the Quadrangle investment group and a prominent Democratic party fundraiser. The Quadrangle Rattner has no connection with the Cosgrove affair.

Credit Suisse said last week that Rattner had resigned to “spend more time with his family”, but the banker acknowledged to The New York Times that he had had an affair with Cosgrove’s wife five years ago and had quit to avoid further aggravation.

“I feel like the star of a bad made-for-TV movie,” Rattner said of Cosgrove’s long-delayed campaign of internet vilification. Cosgrove, an Australian disc jockey and interior designer, declared himself “triumphant”. The resignation brought an apparent end to the bizarre story of a scorned husband who had plastered media websites with salacious accounts of his wife’s supposed activities in London. Cosgrove’s message was always the same. “Steve Rattner paid my wife $500,000 (£260,000) to leave me” read the headline on many of his posts.

In rambling accounts of what happened next, Cosgrove variously accused Rattner of lavishing gifts on the woman he knew as “Kelly Milne”; of taking her on business trips to Macau, Hong Kong, the Philippines, France and Monaco; and of promising her a Ferrari and a house if she stayed with him for at least two years.

It remains unclear why Cosgrove waited several years before launching his attacks on Rattner, who has apologised to his wife and family and is trying to rebuild his marriage. The Cosgroves have since divorced.

Cosgrove said last week that he “had to put my life back together again” and then he had to track down Rattner. It is clear from his early postings that he had confused the Credit Suisse Rattner with the Quadrangle founder, who was obliged to reassure his colleagues that identities had been mistaken.

Both Rattner and the websites that unwittingly hosted Cosgrove’s rants also discovered that there is little legal protection from a determined and vindictive aggressor. The Gawker website suggested last week that Rattner should have sued for libel, but others pointed out that internet libel law remains distinctly hazy – at least in America – and further negative publicity was exactly what the banker was trying to avoid in order to spare his company embarrassment.

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