Explosives were found in Spain Friday under the tracks of a high-speed rail line between Madrid and Seville, near Toledo, Spanish police said.
Bomb disposal experts disarmed the bomb, reports said.
Early reports say the explosives appear to contain the same type of dynamite used in the March 11 Madrid bombings that left 191 people dead and 1,800 injured, which suggests that the same Al Qaeda-linked Moroccan extremist group could be to blame.
The national news agency Efe quoted police sources as saying preliminary analysis suggested the explosives might be a Spanish brand called Goma 2 Eco.
Police have been combing the area where Friday's bomb was placed for members of the Moroccan cell who may have been responsible for the March 11 bombings.
A train employee called in bomb-disposal experts, who discovered a suspicious bag that had 22-24 pounds of dynamite, 430 feet of cable and a detonator inside, Interior Minister Angel Acebes (search) said.
The track runs about 40 miles south of Madrid on the rail line running to Seville, which is popular with beach-goers.
The train line was stopped and remains frozen, keeping 10 trains at a standstill on the rails. Vacationers began leaving town for the Easter holidays Friday, so the stations were packed with travelers. The state rail company RENFE said no train was close to the bomb when it was found.
The track is used by Spain's Ave trains, which can reach speeds of 186 mph.
The discovery follows the March 11 series of 10 backpack explosions that tore through four Madrid (search) commuter trains. Those bombs were detonated with cell phones attached to the explosives.
Fox News military analyst Maj. Bob Belvelacqua said so far, Friday's bomb doesn't look like the word of Al Qaeda, which is known for planning intricate, coordinated attacks.
"This is likely the work of a single bomber," Belvelacqua said.
On Thursday, police in northern Spain defused three letter bombs addressed to journalists in Madrid.
Acebes said the origin of the letter bombs has not been determined, although the mechanism of the bombs is "similar to those that have been used by anarchist groups on previous occasions."
Some March 11 Suspects Freed; Warrants Out for Others
Also on Friday, a judge freed three men who had been arrested in connection with the March 11 bombings, court officials said.
Judge Juan del Olmo ordered the release of two Syrian detainees, Walid Altaraki and Mohamad Badr Ddin Akkad, and Moroccan Fouad Almorabit.
The officials also said witnesses to the March 11 attacks had recognized three jailed suspects at a prison lineup Thursday night. News reports said two of those recognized were Jamal Zougam, a Moroccan prime suspect, and Basel Ghayoun, a Syrian. Both have been charged with mass murder.
Sixteen people are in custody, 14 of whom have been charged.
Del Olmo has issued international arrest warrants for five Moroccans and a Tunisian, Sarhane Ben Abdelmajid Fakhet, described as leader of the group suspected of carrying out Spain's worst terrorist attack. The attack killed 191 people and injured more than 1,800.
The warrant for Fakhet does not suggest he was overall organizer of the attacks. Nor does it give many details about his alleged role.
The court documents said Fakhet, 35, had been an active campaigner for "jihad," or holy war, among some of the suspects already in custody, and as early as mid-2003 had shown signs of preparing a violent act in Spain — specifically the Madrid area — "as a demonstration of the said jihad."
The warrant said Fakhet had helped arrange the rental of the house outside Madrid where investigators say the bombs were assembled, and that four others among the six on the warrant list had been at the house.
Altaraki and Ddin Akkad, the Syrians released Friday, told the judge they knew Fakhet but had no direct ties to him.
Almorabit had been detained and released earlier in the week but was brought back to the court to clear up an unspecified point in his testimony. Almorabit shared a flat with Ghayoun.
He was ordered to appear before the court periodically while the Syrians must keep the court informed of where they live.
The government said Tuesday the investigation was focused on the Moroccan Islamic Combatant Group, forerunner of Salafia Jihadia, which Morocco blamed for bombings in Casablanca that killed 33 people and 12 suicide bombers last year.
But the arrest warrants made public Thursday did not mention the group.
Two people, Abu Musab al-Zarqawi and Abdelkrim Mejjati, have been named as possible top organizers. Neither was among the six names on the international warrants.
Besides Fakhet, those named in the warrants were Moroccans Jamal Ahmidan, alias El Chino; Said Berraj; Abdennabi Kounjaa, alias Abdallah; Mohammed Oulad Akcha, and his brother Rachid Oulad Akcha.
French investigator Jean-Charles Brisard said last week that Spanish officials saw al-Zarqawi, a Jordanian linked to Al Qaeda, as the brains behind the bombings.
Spanish news media have also quoted Moroccan intelligence sources as saying Mejjati, a Moroccan, was the on-the-ground organizer and had been in Madrid three days before the train attacks. But Moroccan authorities told The Associated Press it was not clear what role he had played in the bombings.
The warrants made only one mention of Al Qaeda. They said Berraj met with three Al Qaeda suspects in Istanbul in October 2000. They said he also had ties with the jailed Syrian Ghayoun.
Fox News' Michael Moffett and The Associated Press contributed to this report.