We don't have direct data on whom planted the bombs in the trains, Bogey780. Unlike London or 9/11, no video shows the bombers boarding the trains.
However, we know that Spanish
high rank police officers have allegedly lied, and now face up to six years in prison, to cover-up not ETA's role, but a simple comment concerning ETA on a report in the 3/11 case.
It is highly unlikely, that they, and Zapatero's government, keep silence if the real perpetrators were other Islamists. No such cover-up, that reaches even the Anglosphere, would have been necessary.
Therefore, the truth is far worse.