Billionaire Elon Musk said on Monday in a filing that he will be the chief executive officer of Twitter, the social media company he recently acquired for $44 billion. The move comes after Musk, who also runs Tesla Inc. and SpaceX, fired Twitter CEO Parag Agrawal and other top company officials last week. Musk previously changed his Twitter bio to “Chief Twit” in a sign alluding to this move. Last week, Musk’s takeover of the social media company for $44 billion concluded a months-long saga. Since then, Musk has moved quickly to begin making changes at the company, which he had ridiculed for months for being slow to introduce product changes or take down spam accounts. Musk’s teams began meeting with some employees to investigate Twitter’s software code and understand how aspects of the platform worked, according to two sources familiar with the matter. Some staff who spoke with Reuters said they had received little communication from Musk or other leaders and were using news reports to piece together what was happening at the company.