"Non-consensual sexual deepfakes of women and children are a violent, unacceptable form of degradation," EU Commission Vice President Henna Virkkunen said on Monday.
The investigation will probe whether X has complied with its obligations under European Union law, including the duty to "diligently assess and mitigate systemic risks," the commission, which acts as the EU's digital watchdog, wrote in a statement.
"This includes risks related to the dissemination of illegal content in the EU, such as manipulated sexually explicit images, including content that may amount to child sexual abuse material," it said.
"These risks seem to have materialised, exposing citizens in the EU to serious harm."
Elon Musk's social media platform has come under heavy criticism in recent weeks after Grok allowed users to digitally replace women's clothing with bikinis and, in some cases, create sexualised depictions of minors.
After prolonged international outrage, X introduced some restrictions in mid-January.
These include preventing Grok "from allowing the editing of images of real people in revealing clothing such as bikinis" and restricting the image creation feature of the AI chatbot to paid subscribers, X said in a statement.
The commission stressed that the opening of a probe does not prejudge its outcome.
British media regulator Ofcom also launched an official investigation into Musk's online platform earlier in January over the sexualised images created by Grok.
British Prime Minister Keir Starmer called the AI-created images "absolutely disgusting".
Authorities in France have also been investigating Grok since last year.
The original investigation there revolves around allegations that algorithms on the social network were altered to give more attention to far-right content. New allegations of Holocaust denial and sexual imagery have since been added.
Grok has already been banned in Indonesia and Malaysia.
On Monday, Brussels also announced an extension to a separate investigation into X which was opened in December 2023.
The commission has been working to assess whether the platform's recommender system, the algorithm which suggests content to users, "has properly assessed and mitigated all systemic risks" under the bloc's Digital Services Act (DSA).
The probe now also covered "the impact of its recently announced switch to a Grok-based recommender system", the commission said.
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