The European Commission is investigating X’s handling of illegal content related to the Israel-Gaza war, marking the most significant action the bloc has taken to date under a landmark social media law that requires tech platforms to address illegal content and disinformation.

Thierry Breton, a European commissioner, announced on Thursday that the European Union had sent a request for information to X, the company formerly called Twitter, under the Digital Services Act. The demand marks an escalation from earlier this week, when Breton pressed X owner Elon Musk earlier this week on whether the platform was being used to disseminate “illegal content & disinformation” related to the Hamas attacks against Israel.

“The #DSA is here to protect both freedom of expression & our democracies — including in times of crisis,” Breton wrote.

The European Union’s law, which came into force for large platforms in August, could pose a significant check against Musk and the hands-off approach he has taken to policing misinformation and other harmful content on X. The E.U. has built a reputation as Silicon Valley’s top cop, and X’s handling of terrorist content and other posts related to the war pose an early test of how the law may be enforced.

The EU decided to open the probe into X’s handling of content regarding the Israel-Gaza war after Breton said that X responded to his Tuesday letter, which said the European Commission had “indications” that the platform was being used to disseminate illegal content. He told Musk that the company needed to be transparent about what content was permitted under its rules and that the company needed to be timely in taking action when it receives notices of illegal content in the EU.

Breton also told the company it needed “proportionate and effective mitigation measures to tackle the risks to public security and civic discourse stemming from disinformation.” He cited media reports that repurposed old images of unrelated conflicts and footage from video games was being used to mislead the public about the Israel-Gaza war.

Musk responded publicly to Breton in a post on X on Tuesday.

“Our policy is that everything is open source and transparent, an approach that I know the E.U. supports,” he said. “Please list the violations you allude to on X, so that the public can see them. Merci beaucoup.”

Breton sent warnings to multiple social media companies about their handling of illegal content and disinformation related to the Hamas attacks against Israel under the Digital Services Act.

On Thursday, he sent a letter to TikTok CEO Shou Zi Chew, saying that the company has a “particular obligation” to protect children and teenagers using the social network from “violent content depicting hostage taking and other graphic videos.” In a Wednesday letter to Meta CEO Mark Zuckerberg, Breton asked him to be “very vigilant to ensure strict compliance with the DSA rules.”

Earlier this summer, Breton traveled to X’s headquarters in San Francisco to test whether the company was prepared for its obligations under the DSA.

And shortly after Musk took over X, the world’s richest man tweeted: “The bird is freed.” Within hours, Breton responded that in Europe, “the bird will fly by our rules.”