One of the groups being looked at as possibly responsible for the Madrid bombings is known as the Brigade of Abu Hafs al-Masri, a splinter organization of al Qaeda. But a definite link has not been proven.
It's still unclear who is responsible for the attacks.
The Brigade of Abu Hafs al-Masri sent an e-mail to London-based Arabic language newspaper al-Quds, claiming responsibility for the Madrid attacks. "We have succeeded in infiltrating the heart of crusader Europe and struck one of the bases of the crusader alliance," said the letter, according to Reuters news agency.
The group said it carried out the attack to settle old accounts with Spain, which supported the U.S.-led invasion of Iraq last year. The letter also said the group was 90 percent ready to carry out a major attack in the U.S.
Spanish authorities on Thursday also found a deserted van containing detonators and a tape in Arabic in a town near Madrid, where the bombs may have been placed on the commuter trains. The attacks have so far killed 198 people and injured 1,430. With 60 people still in serious condition, the death toll was expected to rise on Friday.
The Islamic extremist group has also tried to claim responsibility for other disasters in the last year. Some of them were not credible, however: For example the blackouts in the eastern United States last August, which officials showed to be due to technical failure.
In other claims, possible connections are still unclear: Abu Hafs al-Masri said it helped bomb a Jakarta hotel, the U.N. offices in Baghdad, and most recently two synagogues in Istanbul. The group says it is affiliated with terror network al Qaeda. Abu Hafs was the name of Al Qaeda's former military commander whom the U.S. killed in an Afghanistan air strike in November 2001.
Germany ponders al Qaeda connection
German Interior Minister Otto Schily
While the German government said the security situation had not changed after the attacks, Interior Minister Otto Schily (SPD, photo) said the situation would have to be re-evaluated should al Qaeda be responsible. Schily added, however, that the Basque terrorist organization ETA seemed a more likely perpetrator of the attacks so far.
In the wake of the Madrid bombings, German opposition leaders have also been calling for stronger security measures in the national rail system. The Christian Democrats said that security measures on German trains should be stepped up similar to those at airports.
They called for a general overhaul of the current counter-terrorism measures in Germany. And the head of the German police union spoke of a constant threat -- not from ETA, but Islamic terrorism, which threatens Germany as well as Spain.
Bavarian Interior Minister Günther Beckstein (CSU, photo) said he believed al Qaeda could be responsible for the attacks. He added that Germany was equally vulnerable as the country has sent soldiers to Afghanistan. "Especially since Afghanistan is Taliban core country, we're on the radar screen of fundamentalists," Beckstein told Deutschlandfunk radio.