Former Director-General of Spanish security, Julian Sancristobal, was sentenced to 10 years' imprisonment last July for involvement in a "dirty war" against the Basque separatist movement in the 1980s.

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But he was released on a partial pardon in December, along with several other leading Socialist Party figures from the 1980s convicted on the same charge.

Mr Sancristobal is now being investigated in connection with the killing of Santiago Brouard, a leader of Herri Batasuna, the political wing of the armed Basque separatist group, ETA.

Dr Brouard was killed with a sawn-off shotgun at the medical clinic where he worked in the Basque city of Bilbao.

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He was one of many Basque separatists thought to have been killed by an illegal group known as the Anti-terrorist Liberation Group, or Gal.

BBC Correspondent Daniel Schweimler in Madrid says Gal is thought to have operated with the knowledge and possible co-operation of members of the Socialist government of then Prime Minister Felipe Gonzalez..

Bilbao Judge Jose Luis Armengol said Mr Sancristobal supplied money from a secret government fund to pay for the killing, the Spanish news agency Efe reported.

The judge also ordered the detention of a former Civil Guard lieutenant, Rafael Masa, who he said engaged people to carry out the killing, and another former police officer.

Other leading socialists jailed, then freed, last year include former Interior Minister Jose Barrionuevo.

Judge Armengol ordered Mr Sancristobal to be returned to jail without bail as a precautionary measure while he went ahead with his investigation.