Three Algerians who are considered close to a suspected ringleader of the Madrid (search) train bombings are being questioned about an alleged plot to kill judges investigating Islamic terrorists, officials said Thursday.

The three are among 10 prisoners who have been isolated from other inmates for questioning over a possible link to the alleged plot targeting the National Court (search) with a homicide truck bomb packing 1,100 pounds of explosives, police said.

In 2001, the court convicted the three of belonging to an Algerian terrorist group along with Allekema Lamari (search), who was released from jail in 2002 in what officials said was an administrative mistake. Lamari is now suspected of having been a ringleader of the March 11 train bombings in Madrid, which killed 191 people.

Lamari has been identified as being among seven suspects who blew themselves up April 3 as police investigating the bombings prepared to arrest them in an apartment outside Madrid.

A police spokesman declined to comment on Lamari's relationship with the three isolated inmates. But the newspaper La Vanguardia quoted one investigator as saying: "It is not that the three knew Lamari. Rather, they were his people."

The alleged leader of the court bombing plot, Mohamed Achraf (search), is said to have recruited cell members while jailed in Spain for credit-card fraud. Eight suspected cell members were arrested in Spain this week on the basis of testimony from an informant who was in contact with Achraf.

One of those eight detainees, Madjid Sahouane, was arrested in Spain in September 2001 on suspicion of links to terror suspects elsewhere in Europe, but a judge freed him on bail a month later and threw out the case in February of this year, citing lack of evidence.

In 2001, Spain's government had said those suspects included Nizar Trabelsi, a Tunisian identified as being part of a foiled plot that year to bomb the U.S. Embassy in Paris. Trabelsi is now serving a 10-year sentence in Belgium over a plot to bomb U.S. military personnel and involvement in an Al Qaeda linked terror ring in Europe.

Spanish officials had also said Sahouane was also linked to Jerome Courtailler, a French convert to Islam who was acquitted in 2002 in a trial in the Netherlands in the Paris embassy plot. An appeals court overturned the acquittal and sentenced him to six years, and Courtailler surrendered to Dutch police in June of this year.

At the National Court -- the reported target of the newly revealed suicide bombing plot -- Judge Baltasar Garzon was expected to issue an international warrant spelling out charges against Achraf. This would set the stage for Spain to request his extradition from Switzerland, where Achraf was arrested last month for immigration offenses.

Garzon is expected to start questioning the 10 isolated inmates on Friday.

Officials said Thursday that Lamari was released from prison in 2002 because of an administrative mistake. Chief Justice Francisco Jose Hernando of the Supreme Court has launched an investigation into what went wrong with the release, a court official said.

Lamari was first jailed in 1997 on suspicion of belonging to an Algerian terrorist group.

At his trial in 2001, he was sentenced to 14 years in prison by the National Court. Lamari appealed to the Supreme Court. In April 2002, the National Court reduced his sentence to five years and said it would release him on June 29, 2002, counting his time served since 1997, unless the Supreme Court ruled on the appeal.

Even though the Supreme Court rejected the appeal June 7, the documents took a month to reach the other court, by which time Lamari had been released.

Spanish intelligence agents warned police in November 2003 that Lamari was preparing an attack in Spain, the newspaper El Pais reported earlier this week. The agents asked the Interior Ministry for help in locating him, but the ministry did not heed the warning, El Pais said.