MADRID, June 13 ? For the past three months, more than two dozen defendants have been on trial in Madrid on charges that they orchestrated one of the worst terrorist attacks in Europe’s recent history ? the bombing of four commuter trains on March 11, 2004, which killed 191 people and wounded at least 1,800. But at many points during the trial, the courtroom has been absorbed by a sideshow ? the effort by a handful of lawyers for some victims to demonstrate that ETA, the armed Basque separatist group, had a part in the bombings.

The efforts to implicate ETA have muddied an already complex trial and undermined the credibility of the police and judges involved in the investigation, members of the prosecution and some victims said. The efforts also show the degree to which politics has reached into the corners of public life.

“A parallel trial has emerged, based on unfounded suspicions and preconceived ideas,” said Javier Zaragoza, the chief prosecutor, on Monday. He said the insistence by some lawyers on pursuing the ETA theory had led to “grotesque situations? in the courtroom.

That the theory linking ETA to the attacks should be a prominent feature of the trial was no surprise, legal and political analysts have said. The decision of the former government of Jos