OVIEDO, Spain

One of two Spanish intelligence agents arrested for sending classified information to the US was head of the Russia division of Spain's National Intelligence Center (CNI), El Pais reported Monday.

According to the Spanish daily, the US infiltrated the CNI to uncover additional intelligence, besides that shared routinely between the two allied nations.

Washington sought confidential information on Madrid's connections with Moscow and its insights into Russian networks.

Spanish media has reported that it was the most significant known intelligence breach in the CNI in nearly 30 years.

The two agents were reportedly arrested in September for sending the information across the Atlantic.

One was a seasoned agent who El Pais reports had been working in the Navy for decades and led Spain's Russian intelligence division. He is still in pre-trial detention.

The other, his subordinate, has been released on bail.

The investigation is ongoing, according to El Pais, as authorities seek to determine the motives for sending the US this information, which would affect the charges placed against the agents, possibly ranging from treason to revealing state secrets.

Spain had tried to keep the scandal largely under wraps. When the leaks were caught, Spain's defense minister quietly summoned the US ambassador.

At least two US intelligence agents working in the US Embassy were also expelled from the country for their roles in infiltrating Spain's intelligence service, El Pais reported.

While US Ambassador Julissa Reynoso Pantaleon has not publicly commented on the intelligence issue, El Pais reports that she blames the spying on a secret program from the era of former President Donald Trump.

Spain's Defense Minister Margarita Robles has also spoken very little on the case.

When asked, she emphasized that Madrid and Washington are "friends, partners, and allies" and that the spying does not change that.

"When there are issues, they are discussed and dealt with, but that does not influence the relationship that we have," she told journalists last December.