(Bloomberg) -- The US Securities and Exchange Commission sent a query last month to Elon Musk over how he disclosed his major stake in Twitter Inc. -- the clearest signal yet that the Wall Street regulator is scrutinizing his efforts to buy the social network.

The SEC requested information from Musk about the timing of his disclosure and the type of filing he made, according to a letter dated April 4 and posted by the SEC. The regulator asked why Musk “does not appear to have been made within the required 10 days from the date of acquisition as required.”

Musk disclosed on April 4 that he acquired more than 9% in the company, a week later than regulations allow and by using a filing typically reserved for passive investors. He has since embarked on a highly-public takeover bid.

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