Spain's intelligence service is "99 percent certain" that radical group and not the Basque separatist group ETA are responsible for train bombings that killed 200 people, a Spanish radio station said on Saturday.
Private radio SER, whose owners have links to the opposition Socialist Party, said the National Intelligence Centre (CNI) believes the evidence points to an Islamic group, and that 10 to 15 people left bombs on the trains and fled, the radio said.
"The evidence has wiped out previous indications that led us to believe in ETA," the radio quoted one of its sources as saying.
Who was responsible is seen by political analysts as an important question ahead of Sunday's general election.
Interior Minister Angel Acebes told a press conference: "If it (is believed to be) al Qaeda, no authorities from the state security forces have told me." A Defence Ministry spokesman declined comment on the radio report.
Many analysts say any proven ETA involvement in the bombings would probably benefit the ruling party.