The leader of the Netherlands’ ruling Labor Party, Diederik Samsom, is suspected of disclosing information from a classified intelligence briefing one year ago. The speaker of the European nation’s parliament proposed on Thursday to appoint a committee to investigate the leak.

The speaker was notified a day earlier by the public prosector that it could no longer carry on the investigation because a member of parliament was likely involved. Lawmakers are immune from prosecution.

Halbe Zijlstra, Samsom’s liberal party counterpart, triggered the investigation when he filed charges with police in early 2014 after NRC Handelsblad, a newspaper, had published details from a 2013 meeting of parliament’s intelligence oversight committee.

Zijlstra, as the leader of the largest party, chairs the intelligence committee. His party rules in coalition with Labor.

Samsom is a suspect because the author of the NRC Handelsblad story earlier wrote a book about the Labor Party leader.

The leak also benefited Samsom’s interior minister, Ronald Plasterk, whose department is responsible for the security services.

When foreign media reported in 2013 that some 1.8 million Dutch phone records had been gathered and shared with the American National Security Agency, Plasterk initially denied this. He was later forced to admit the reports were broadly accurate after all.

In December 2013, Plasterk briefed the intelligence committee but it is unclear what he told them behind closed doors.

Some of the party leaders who attended the meeting later suggested parliament still hadn’t been adequately informed about the phone data. The opposition liberal Democrats called a confidence vote in Plasterk — which he survived. Then NRC Handelsblad reported Plasterk had actually told he committee what it needed to know.

A story in the conservative weekly Elsevier at the time fingered Samsom as the leak.

The Labor Party leader denies this.

The same Elsevier story said the Netherlands had access for years to signals intelligence gathered by the “Five Eyes” alliance between the United Kingdom, the United States and other English-speaking nations but was dropped from the network when it pulled out of the NATO mission in Afghanistan in 2012; a decision that happened as a result of Labor’s opposition to the war.