Tesla CEO Elon Musk bought a 9.2% stake in Twitter Inc., according to an SEC filing posted Monday.

Musk purchased roughly 73.5 million shares, making him the largest shareholder, The Associated Press reportedMonday, citing the filing.

Musk has questioned whether Twitter “rigorously adheres to” the principle of “free speech.” Musk posted a poll on Twitter and 70.4% of respondents said Twitter does not.

Musk also said he was “giving serious thought” to creating a new platform with “free speech.”

The investment is considered passive, according to the AP.

Dan Ives of Wedbush Securities said in a client note Monday that the passive stake is likely “just the start of a broader [conversation] with the Twitter board/management that could ultimately lead to an active stake and a potential more aggressive ownership role of Twitter,” according to the AP.

Twitter shares spiked more than 20% following the announcement.