BRUSSELS, March 12: Speculation that Thursday's bomb attacks in Madrid may have been the work of an extremist Muslim group mounted across Europe on Friday after a little-known shadowy organization claimed responsibility for the fatal explosions in the name of Al Qaeda.

Spanish Prime Minister Jose Maria Aznar initially blamed the Basque separatist group ETA for the terror attacks which killed almost 200 people and injured more than 1,400 - and which newspapers are already describing as "Europe's 9/11," a reference to the attacks on US landmarks in 2001. But on Friday, Mr Aznar told reporters that no suspects had been ruled out.

"We will bring the guilty to justice," Mr Aznar told a news conference.

"No line of investigation is going to be ruled out." As Spain started three days of mourning, campaigning in Spain's general election on Sunday was called off but Foreign Minister Ana Palacio pledged that the vote would be held as planned.

Fears that the Madrid tragedy was the work of Muslim extremists mounted after the Arabic newspaper Al-Quds al-Arabi said it had received a claim of responsibility issued in the name of Al Qaeda.

The e-mail claim, signed by the shadowy Brigade of Abu Hafs al-Masri, was received at the newspaper's London offices and said the brigade's "death squad" was settling old accounts with "Spain, the crusader, and America's ally in its war against Islam." But experts said Al-Masri sometimes falsely claims to be acting on behalf of Al Qaeda.

However, the Spanish government said it was convinced ETA was behind the blasts. Experts said ETA had tried a similar attack on Christmas Eve.

The Interior Ministry also said tests showed the explosives used in the attacks were a kind of dynamite normally used by ETA. Newspapers across Europe, however, speculated about the Al Qaeda connection, warning that all European countries may be vulnerable to similar attacks.