Investigators have said the March 11 bombings were carried out with Spanish dynamite - Goma 2 Eco. The attack killed 191 people and injured 1,800.
On Friday morning, authorities immediately stopped six bullet trains using the Madrid-Seville line after the discovery of the 26-pound bomb under a track about 40 miles south of Madrid. The bomb failed to detonate because it wasn't properly connected, officials said.
No train was near the site when the bomb was discovered, state rail officials said. But about 1,600 passengers left their trains and were taken to their destinations by charter buses. Train service was restored Saturday.
"The explosives (found Friday) are the same as those used on March 11, but at this moment the investigation is continuing ... to try to determine who is behind it," Acebes told reporters, confirming an initial analysis.
Also Saturday, the Spanish newspaper El Mundo reported that the Spanish Embassy in Egypt received a letter from an Islamic militant group threatening new attacks if Spain doesn't withdraw its troops from Iraq and Afghanistan.
In the letter, The Brigade of Abu Hafs al-Masri, a group that also claimed responsibility for the March 11 attacks, threatened to strike against Spanish diplomatic missions in North Africa and the Mediterranean region unless Spanish troops are withdrawn in four weeks.
A Spanish diplomat in Cairo, speaking on condition of anonymity, confirmed that the embassy received a threatening letter signed by Abu Hafs after the commuter train attacks last month.
The United States believes the Abu Hafs group lacks credibility and has only tenuous ties to al Qaeda. In the past, the group has claimed responsibility for events to which they were not connected.
The discovery of the bomb on Friday rattled a country still traumatized by the March 11 attacks in Madrid, which officials believe may have been carried out by an Islamic extremist group from Morocco. Authorities on Friday stepped up security on Spain's entire rail network and used soldiers to provide surveillance.
Judge Teresa Palacio, a magistrate at the National Court, said Friday there was no evidence pointing to either the armed Basque separatist group ETA or the al Qaeda terror network in the failed attack. ETA has targeted Spanish rail lines in the past.
Because the bag was dry and the ground was wet, authorities believe it was placed at the scene Friday. A 450-foot-long cable attached to a detonator looked new.
Acebes said Friday the bomber or bombers may have been scared away by security guards as they planted the device. There was no warning about the bomb, he said.
The line where the bomb was found mainly serves Spain's AVE bullet trains, which have a maximum speed of 190 mph, although some slower trains also use it.
Spain's entire rail system will get a new security system employing helicopters, four-wheel-drive cars and armored vehicles from the Spanish army.
Also Friday, a Spanish judge charged a 15th suspect in the March 11 bombings on four commuter trains, Spain's worst terror attack. The government has said its investigation is focused on the Moroccan Islamic Combatant Group, which has links to al Qaeda.
Six of the 15 suspects detained in the March 11 bombings are charged with mass murder, while the remaining nine are accused of collaborating with or belonging to a terrorist organization.
Otman El Gnaout, a Moroccan, was jailed Friday on terrorism charges. Eleven of the 15 suspects are Moroccan.
Judge Juan del Olmo, the investigating magistrate, has issued international arrest warrants for five Moroccans and a Tunisian, identified as Sarhane Ben Abdelmajid Fakhet andsuspected of carrying out the attacks.