MADRID — A judge in Madrid has opened an investigation into two agents of Spain's National Intelligence Centre for allegedly passing secret information to the United States, the country's defence minister said on Monday.

They were arrested last September and sent to prison after being brought before a judge, Spanish daily El Confidencial confirmed.

Defense Minister Margarita Robles said the intelligence agency itself had filed a complaint with the court. Robles, who was visiting a military base in Latvia, declined to give further details.

What is known so far is that those under investigation allegedly provided Washington with operational data classified at the highest level.

The Centre noticed that two of its agents were consulting information that was not related to the tasks assigned to them.

The theft of classified documents was then discovered during one of the Centre's regular checks of its staff.

It is not yet known how much information may have been passed to the US agents, but the investigation has revealed that the information was exchanged for money.

The investigating judge decided to send them directly to prison to prevent the destruction of evidence.

One of the detainees remains in prison, as he is said to have played a more prominent role, while the other has been released on bail.

The Spanish Penal Code punishes any Spaniard who discloses classified information with the intention of favouring a foreign power with six to twelve years in prison.

The newspaper El Confidencial said that these arrests "would have provoked a crisis with Washington, although the government of Pedro Sánchez has tried to keep it quiet". — Euronews