The Spanish judge investigating the Madrid train bombings on 11 March has issued international arrest warrants for five suspects.
One of the bombing suspects was released and rearrested
The warrants are being forwarded to authorities in Britain, Morocco and Germany, according to reports.
One is for the Moroccan national, Abdelkrim Mejjati, the alleged mastermind of the attacks.
Spain's Interior Minister Angel Acebes has named an Islamist extremist group as a focus of the investigation.
Mr Acebes said the Moroccan Islamic Combatant Group was the "investigators' priority".
But Mr Acebes insisted that other terrorist organisations had not been ruled out.
He said investigators were in touch with a number of police forces, "but most directly with Germany, Britain and Morocco".
Contrary to reports from Britain, Mr Acebes said Spain had information from British security services suggesting there was a possible link to one of eight people arrested in London after the discovery of a large store of explosive fertiliser on Tuesday, said French news agency AFP.
The head of Britain's Metropolitan Police anti-terrorist branch, Peter Clarke, had said there is no link between the Madrid bombings and the eight London suspects.
The other warrants are for men believed to either have been among the perpetrators or to have played a supporting role.
Twenty suspects are being held over the attacks, which killed 191 people.
Police arrested Otman El Gnaout on Tuesday in connection with the investigation but his age and nationality have not been revealed.
One of three men released by the judge on Tuesday was rearrested on Wednesday for further questioning, say reports from Spain.
Moroccan Aughar Fouad El Morabit was first arrested last week in Ugena, near Toledo, and is said to have shared a flat with Syrian-born Basel Ghayoun who was identified by a number of victims of the train attacks.
Abdelkrim Mejjati has also been linked with multiple bombings in the Moroccan city of Casablanca last May, which killed 45 people.
The BBC's Katya Adler in Madrid says the Spanish investigation suspected a Casablanca connection from an early stage.
She says Moroccan investigators claim they are focusing on the group blamed for the Casablanca bombings, Salafia Jihadia, in connection with the Madrid attacks.
The group is an offshoot of the Moroccan Islamic Combatant Group.
According to the US State Department, the Moroccan Islamic Combatant Group emerged in the late 1990s.
It says the group's aims include establishing an Islamic state in Morocco and that it supports al-Qaeda's "holy war" against the West. It says the group is based in Afghanistan, western Europe and possibly Morocco.
Reports suggest the Moroccan government has also been investigating the group.
Ten of the suspects in Spanish detention are thought to be Moroccan.
Fourteen of the suspects detained now face provisional charges over the attacks, which allow them to be held for up to two years while investigators gather evidence.
On Monday, three more people were arrested, bringing the total number of suspects in police custody to 18, out of a total of 23 detained during the investigation.