Spanish police have arrested three Algerians and a Moroccan over an alleged plot by an Islamist cell to blow up a court and other buildings.
Suspect Faisal Allouch, who was also held over the Madrid bombings
The three Algerians, who were not immediately identified, were arrested in the south-eastern city of Gandia.
The Moroccan, named as Faisal Allouch, was detained in Madrid.
He was among several suspects arrested over the Madrid train bombings in March this year, in which 191 people died, but he was later released.
The arrests raise to 33 the number of suspects detained in the investigation being carried out by the leading judge, Baltasar Garzon.
One of the targets of the alleged plot was the High Court in Madrid - where investigators into the train attacks of 11 March are based.
The plot is said to have involved detonating a lorry loaded with 500 kg (1,100 lb) of explosives near the High Court and Supreme Court. No weapons or explosives have been found.
The High Court, base of the investigation into the Madrid bombs
Thirty arrests of alleged members of the Martyrs for Morocco cell have already been made in the last two weeks. One was reportedly released.
Mr Allouch was jailed in late March after being one of the first suspects arrested over the Madrid bombings. On 5 May, he was released by a judge who ordered he report to court daily.
An Algerian man being held in Switzerland, Mohamed Achraf, is fighting extradition to Spain to face charges of setting up the alleged Martyrs for Morocco cell.
Around 20 suspects are currently being held in custody over alleged links to the March attacks on commuters in Madrid.
A further seven were killed when their explosives blew up as police approached their hide-out in Madrid.