The sentences ranged from 3 to almost 43,000 years, although under Spanish law, the maximum anyone is forced to serve is 40 consecutive years. One defendant was released during the trial for lack of evidence.

Many Spaniards were shocked that the focal suspects were not convicted of the most severe charges.

The verdicts closed a sprawling trial that over the course of five months brought 29 defendants, nearly 50 lawyers and 350 witnesses to a temporary courtroom on the outskirts of Madrid.

The trial promised the first taste of justice to those wounded in the attacks and the relatives of those killed on March 11, 2004, when blasts from 13 sports bags stuffed with explosives and nails tore through four trains carrying people from mainly working-class suburbs to the city center.

Those who believed that prosecutors had produced enough evidence to convict the main suspects of the most serious charges were disappointed.

Isabel Presa, who lost her youngest son in the blasts, told reporters outside the courtroom, “I’m not a judge or a lawyer, but this is shameful, outrageous.”

According to Reuters, Ms. Presa said the attacks had “condemned me and my husband to a life sentence, and these people get off scot-free.”

Counterterrorism experts said the verdict underscored the difficulty of building a solid case against people accused of inspiring or directing Islamist foot soldiers, and who belong to diffuse groups with little formal structure.

The bombings were carried out by a group of North African Islamists that intersected with a band of petty criminals whose ringleader, Jamal Ahmidan, had become radicalized in a Moroccan jail. Seven of the main suspects, including Mr. Ahmidan, blew themselves up in a Madrid apartment when they were surrounded by the police three weeks after the attacks, and four others are believed to have fled.

Without a case strong enough to convict those suspected of being organizers, the prosecutors failed to prove a connection between the group that carried out the attacks and international Islamists with links to established organizations, like the Moroccan Islamic Combatant Group.

The counterterrorism experts said the verdicts reflected the challenges faced by police forces and judges as they seek to imprison those accused of international terrorism: the preponderance of circumstantial evidence rather than concrete proof; problems with evidence translated from Arabic and with evidence collected by other countries; unreliable witnesses; and the absence of confessions — none of the 28 defendants confessed.

“It is a point of pride to be able to try people in a courtroom, with full constitutional guarantees,” Fernando Reinares, an expert in international terrorism at the Royal Elcano Institute, said. “But in Spain there is space for debate about whether we need to adapt our judicial legislation and culture to confront international Islamist terrorism.”

Roland Jacquard, head of the International Observatory on Terrorism in Paris, said prosecutors had encountered similar difficulties in countries like Germany, where people accused of complicity in the Sept. 11, 2001, attacks on the United States were acquitted for lack of evidence.

He said: “We need to find a legal formula that would give evidence of the masterminds’ responsibility, and not only of the responsibility of the operatives. It is always easier to arrest someone who has imprints of explosives on his hands.”

Javier Gómez Bermudez, the presiding judge on the tribunal, sentenced Jamal Zougam, 34, a Moroccan whom witnesses saw on one of the trains that was later bombed, to more than 30,000 years in prison for charges including murder. Mr. Zougam owned a shop where most of the phone cards used in the mobile phones that detonated the explosives were bought.

The tribunal gave a similar sentence to Otman el-Gnaoui, 32, a Moroccan who helped transport the explosives used in the attacks, and to José Emilio Suárez Trashorras, 30, who was convicted as a “necessary accomplice.” Mr. Suárez, a former miner from northern Spain, supplied the stolen dynamite used in the bombings in exchange for drugs.

But the tribunal acquitted Rabei Osman Sayed Ahmed, who was accused of being a March 11 organizer. Last year, he was convicted in an Italian court of conspiracy to participate in international terrorist activities.

The other defendants who were accused of being organizers, Hassan el-Haski and Youssef Belhadj, were acquitted of any such role and convicted of belonging to a terrorist group.

In written arguments released Wednesday, the tribunal said tapes of telephone conversations made by the Italian police and provided as evidence against Mr. Ahmed did not prove his participation in the plot. Prosecutors said Mr. Ahmed was caught boasting that he was “the thread behind the Madrid plot,” but the translation from the Arabic was disputed by Spanish translators in the Madrid court.

The tribunal also said a piece of paper found in Mr. Ahmed’s Milan apartment, bearing the words “martyr,” “honey” and “11-03-04” — the European rendering of the date of the attack — was not conclusive evidence.

Mr. Reinares, the expert on terrorism, said the tribunal appeared to have been very strict in its definition of admissible evidence. “It seems he has not admitted the extraordinary mass of circumstantial evidence,” Mr. Reinares said. “This kind of evidence is crucial when you are trying members of a nebulous group of international terrorists.”