Sweden's Financial Supervisory Authority (FI) said it would investigate to see if OMX used inappropriate defense measures after Borse Dubai launched an unsolicited bid for the exchange owner.
A spokeswoman for the regulator confirmed a report in Swedish daily Dagens Industri that said the investigation was prompted by media reports. "Yes that's correct," said spokeswoman Helena Ostman regarding the report.
She declined to comment on what media reports prompted the investigation or provide any other details on its scope.
"It's an urgent matter. Hopefully, we will have something to say by the end of the week," the spokeswoman told Reuters.
Jonas Rodny, a spokesman for OMX, said the firm had received a letter from the regulator.
"They usually send these kinds of letters in connection with a takeover," he said. "We are preparing the answers at the moment, and we will send them this week to them."
Rodny said this was "nothing dramatic," and he declined to go into detail on the questions asked by the regulator. Asked which media reports might have prompted the questions, he also declined to comment.
Last week, Reuters reported that OMX management had complained to the Swedish Bar Association about a Borse Dubai legal adviser working on the takeover attempt because he had recently left OMX.
Typically, issues regarding takeover bids would be handled by a company's board of directors rather than corporate executives, but OMX said at the time that this was a matter for management, not for the board, as it related to company security.
Borse Dubai, a holding company for Dubai exchange interests, has offered 230 crowns in cash for OMX. Its $4 billion offer topped an agreed cash-and-shares bid from U.S. exchange Nasdaq, valued at 207 crowns per share, or $3.6 billion.
On Monday, Sweden's economic crimes unit said it would not be opening an investigation into Borse Dubai activity concerning OMX. The unit had been considering whether to begin a probe after allegations regarding trades.
"There is no cause to believe that an offence subject to a public prosecution has been committed," the crimes unit prosecutor said on Monday.