US securities regulators have opened an informal investigation into the sudden drop of United Airlines’ shares following the re-publication of a six-year-old story about the carrier’s bankruptcy, people briefed on the matter said.

The inquiry, which is in its early stages, comes as the US Securities and Exchange Commission ramps up efforts to crack down on false rumours intended to manipulate share prices.

“Anytime anyone spreads false information about a public company over a communication medium like the internet, its message boards, chat rooms or otherwise, that will raise questions as to whether someone is committing securities fraud,” said John Reed Stark, head of the SEC’s office of internet enforcement. But he declined to comment on the United situation.

United’s shares lost more than 75 per cent of their value on Monday before trading was halted after an investment newsletter found a 2002 Chicago Tribune article in a Google News search and, believing United had filed for bankruptcy protection for the second time this decade, published a summary of it on Bloomberg’s newswire.

Tribune and Google have blamed one another for their roles in the episode, which began early Sunday morning when a single visitor to the website of a Tribune sister paper, the South Florida Sun-Sentinel, viewed the 2002 story reporting on United’s bankruptcy filing. Tribune has not identified the visitor.

Tribune said the visit was enough to land a link to the story on the Sun-Sentinel’s most-viewed list at that hour. Google said its web-trawling program spotted the link and indexed it as a new story when it could not find a 2002 dateline.

Tribune has said it would have been obvious to a reader that the article was six years old.

The explanations have not brought an end to United’s own investigation. While its shares rallied once trading resumed on Monday, the stock still closed 10 per cent lower that day.

United has considered filing a lawsuit against those companies and individuals it believes were at fault for Monday’s chain of events, and the SEC’s actions may help the airline to obtain data to make its case, people familiar with the company’s plans said. United declined to comment.