The e-mail, sent March 27 from an account at the agency called "workexperience3," contained contractual and salary information for all the company's employees in London and a small New York office. CHI & Partners is 49% owned by WPP Group.
Johnny Hornby, one of the agency's founders, said, "I asked our office services manager to call in the police. We have an obligation to take this seriously. All we know is that it was sent by someone from outside the building who got a log-in using an old password. Whether it was a former employee or a freelancer I can't say."
The e-mail contained files of job-offer letters -- including terms and conditions -- to prospective employees, most of whom now work at the company. Many of the letters were "quite old," Mr. Hornby said.
The directors of the company are all legally obligated to publish their salaries annually at Companies House, a registry of all U.K. limited companies, so that information is already in the public domain.
"I've told staff that they can have a quick look at the e-mail if they want but asked them to delete it immediately afterwards," Mr. Hornby said. "I've made it clear that these were just offer letters, so it's not worth people drawing comparisons and approaching their heads of department."
Mr. Hornby believes there is little chance of identifying the culprit. "I'm not sure we are very high up the list of priorities for the computer fraud squad," He said. "We may never know whether it was someone being an idiot or someone being malicious."