The European Ombudsman has opened an investigation into the secret meetings between European institutions to shape laws, known as trilogues.

Trilogues have become the main tool by which EU legislation comes about, but they take place behind closed doors with very little public information about the negotiation process.

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In a letter to the EU institutions, Ombudsman Emily O'Reilly wrote that the level of transparency of trilogues “has been drawn to my attention by several MEPs, MPs from some member states as well as by business and civil society groups”.

She told journalists on Thursday (28 May) that the investigation will focus on trying to map what documents exist, before putting to the test which of those should be public.

“I can't say at this stage what should be released, because I don't know what is released, what is withheld, or what is even recorded," O'Reilly said.

“It's a mapping exercise”, added Fergal O Regan, who is the lead investigator.

In June and July, he and his colleagues hold an inspection at the European Parliament, the European Commission, and the Council, which represents the member states.

“More than anything, it's to try and find out what we don't know. What documents are there in the files that we're not aware of, which are relevant to the functioning of a trilogue, and the arrival at an agreement”, said O Regan.

The ombudsman sent letters to the three institutions on Tuesday asking them to provide a list of trilogues; which documents are created, and which are or can be published; and in which language the trilogues take place.

“As far as we know now, there are no joint minutes. ... each institutions may have their own minutes. That's one thing we are going to look at,” noted O Regan.

The Ombudsman office will focus its investigation on two files closed in 2014: the Mortgage Credit Directive (on consumer protection for mortgages) and the Clinical Trials Regulation (EU-wide rules on clinical trials for medicines).

The two were chosen because the issue are “in the public interest”.

“We were initially wondering if we should follow a trilogue that's ongoing”, said O'Reilly, naming the trilogue meetings on the future of roaming surcharges in the EU as an example.

“But then we thought that if we did that, our presence might affect the trilogue.”

Once the mapping exercise is done, the Ombudsman will decide if further steps are needed to improve transparency.

O'Reilly noted that a balance needs to be struck between accountability and speed as the trilogue process is popular for its ability to speed up the legislative process.

“A lot of people like them because they are quick, they are efficient,” said the Ombudsman.

In the past five years, there have been 1,500 trilogue meetings. On the morning of the O'Reilly's announcement, a trilgoue resulted in agreement on the EU's new investment fund.

The three EU institutions have until 30 September 2015 to respond to the Ombudsman's questions.