A bomb found under a train track was made of the same type of explosives used in last month's Madrid terror bombings, Interior Minister Angel Acebes said yesterday, as train service was restored on the high-speed rail line.

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

yesterday.

"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to strike against Spanish diplomatic missions in North Africa and the Mediterranean region unless Spanish troops are withdrawn in four weeks.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 such as last summer's blackouts in North

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.