A man from Northern Ireland is among a six-member commission that claims to have witnessed arms decommissioning by the Basque separatist group, Eta.
Chris Maccabe, a former political director of the Northern Ireland Office (NIO), is now a member of the International Verification Commission.
In a video released on Friday, two IVC members were seen inspecting weapons.
On Sunday, Mr Maccabe was among three IVC members summoned to a Spanish court to answer questions about the video.
It is understood that the Belfast man confirmed to a Spanish judge that Eta members had handed in guns and explosives in a first step towards disarmament.
The Basque separatist group, which killed more than 800 people in its 40-year campaign, was designated as a terrorist organisation by the European Union in 2001.
The video footage, which was released to the BBC on Friday, shows masked men displaying a small cache of weapons in the presence of two IVC inspectors.
The Spanish government does not recognise the work of the IVC.
On Friday, Spain's Interior Minister Jorge Fernandez Diaz said Madrid would not give credibility to a "theatrical exercise".
On Sunday, three commission members were summoned to the High Court in Madrid to be questioned about their dealings with the masked militants.
The six-strong IVC, based in Amsterdam, was established two and a half years ago.
Its aim is to provide verification of Eta's January 2011 ceasefire and subsequent declaration of a "definitive end of violence" in October of the same year.
The commission consists of former diplomats, politicians and political advisers, including Mr Maccabe.
The IVC is led by Ram Manikkalingam, who has worked in conflict resolution in Iraq, Northern Ireland and his homeland of Sri Lanka.
Mr Manikkalingam and his IVC colleague Ronald Kasrils were the two weapons inspectors who appeared in the video broadcast on 21 February.
Mr Kasrils is former deputy defence minister of South Africa.
Northern Ireland's second largest political party, Sinn Féin, has had extensive involvement in Eta's attempts to move away from its armed campaign.
Sinn Féin President Gerry Adams has condemned the decision to summon the IVC members to the Spanish High Court "to answer questions on their role in the process of disarmament".
Mr Adams said the summons was a "bad decision prompted by bad laws which have no place in a peace process".