DUBAI (Reuters) -  letter, signed by the Abu Hafs al-Masri Brigades which said it is part of al Qaeda, was sent to the London-based al-Quds al-Arabi newspaper. A copy of the letter was faxed to Reuters in Dubai. It was not possible to independently authenticate the letter. The United States blames al Qaeda for September 2001 attacks on New York on Washington that killed some 3,000 people. The Spanish government has said Basque separatists are the main suspect in the bombings of four trains early on Thursday that killed 192 people. But the country's interior minister held open the possibility of a militant Islamist link on Thursday evening when he told a newspaper conference that a suspect van had been found containing detonators and an Arabic-language tape. He said, however, militant Basque separatists remained chief suspects. The al-Quds al-Arabi newspaper received similar letters from the same brigade claiming responsibility on behalf of al Qaeda for a November bombing of two synagogues in Turkey and the August bombing of the U.N. headquarters in Baghdad.