BRUSSELS (Reuters) -Microsoft's ( MSFT ) $13 billion investment in OpenAI could face an EU antitrust investigation as regulators are building the case for such a move, people familiar with the matter said on Thursday.

Microsoft's ( MSFT ) OpenAI partnership, together with those between Alphabet, Amazon and Anthropic, have triggered scrutiny on both sides of the Atlantic as antitrust enforcers seek to understand how these deals affect competition.

The people said the European Union antitrust regulator had decided not to investigate the partnership under EU merger rules, but that Microsoft ( MSFT ) could still face an antitrust investigation.

That could look into whether the partnership restricts or distorts competition within the EU internal market or whether Microsoft's ( MSFT ) market power distorts the market through certain practices.

The European Commission is leaning towards the second angle, one of the people said.

The people said no decision has been taken as the EU competition enforcer is looking for evidence and may eventually decide not to open an investigation.

Microsoft ( MSFT ) has a non-voting position on the OpenAI board but says it does not own any portion of the ChatGPT maker.

The Commission did not comment on the broader antitrust situation. It said it is checking whether Microsoft's ( MSFT ) investment in OpenAI might be reviewable under the EU Merger Regulation.

"In this respect, it is however important to underline that in order to look into potential competition concerns, the Commission first needs to conclude that there has been a change of control on a lasting basis," a spokesperson said, declining further comment.

Microsoft ( MSFT ) declined to comment.