The Commission said in a statement that it launched the probe in response to “indications received concerning the presumed transmission of illicit content”.
X has until Wednesday to respond to the most urgent questions in the 40-page document and until October 31 for less pressing requests for information.
The move comes after social media platforms including X have featured violent content and misinformation about the Israel-Hamas war, including footage from other conflicts and video games.
X, which has slashed its content moderation team under Musk, has come under particular scrutiny after an EU study found that it was the worst-performing platform for combatting disinformation.
Thierry Breton, the 27-nation bloc’s self-proclaimed digital “enforcer”, earlier this week clashed with Musk online after accusing his platform of allowing the spread of “violent and terrorist content”.
Breton has sent similar nonbinding warning letters to the heads of Meta, Instagram and TikTok.
X chief executive Linda Yaccarino has defended the social media network’s record, pointing to the deletion of hundreds of Hamas-linked accounts and the removal or labelling of tens of thousands of pieces of harmful content.
Under the EU’s Digital Services Act, which came into effect in August, platforms that fail to crack down on content deemed illegal can be fined up to 6 percent of their global turnover or even banned from operating within the bloc.
Fighting between Israel and Hamas entered a seventh day on Friday as the Israeli army instructed more than one million Palestinians trapped in Gaza to move south within 24 hours before an expected ground offensive.
Israeli air strikes on Gaza over the past week have killed more than 1,500 Palestinians and wounded more than 6,600 others, according to Palestinian officials.
More than 1,300 people were killed and 3,000 injured by Hamas in the deadliest attack on Israel in decades on Saturday, according to Israeli officials.