The EU voiced concerns about "harmful" content posted on Snapchat, TikTok and YouTube as it told the digital platforms on Wednesday to hand over more information on how their platforms recommend content to users.
The European Commission, the EU's powerful digital watchdog, said the three platforms must "provide more information on the design and functioning of their recommender systems".
Recommender algorithms are used by platforms to push more personalised content.
Wednesday's query was made under a landmark law known as the Digital Services Act (DSA) that forces platforms to do more to protect users, especially minors.
The EU wants YouTube and Snapchat to give "detailed information on the parameters used by their algorithms to recommend content" and the role they play in "amplifying" certain systemic risks, including those related to minors or users' mental health.
Under the DSA, platforms must mitigate risks arising from such systems.
The commission also wants to know what steps the two platforms have taken to mitigate the influence of their algorithms on the spread of hate speech and illegal drugs.
Meanwhile, the EU demanded TikTok give more information on measures taken "to avoid the manipulation of the service by malicious actors and to mitigate risks related to elections, pluralism of media, and civic discourse".
Snapchat, TikTok and YouTube must fulfil the request by November 15.
A senior EU official said the commission wanted to understand "what kind of controls" the platforms have in place and "how effective these controls are", adding that regulators sought access to "a lot of internal documents on the tests they have done".
The request is a first step in a possible compliance procedure, but does not itself suggest the law has been broken, nor is it a move towards punishment.
The senior official said the EU was "acting on a suspicion" of infringement, but the platforms will have the chance to make changes that address the commission's concerns.
"Often we see companies actually correct themselves after a request for information."
The EU is also looking into content recommender systems in investigations launched under the DSA into TikTok, AliExpress, Facebook and Instagram.
