BLAME: The government, who lost elections held three days after the attacks, initially blamed Basque separatists Eta. But al-Qaeda soon became main line of inquiry.
Detonators and a tape recording of Koranic verses were found inside a white van at Alcala de Henares. Eta denied involvement, while groups linked to al-Qaeda claimed responsibility in a letter and video message.
LEADS: The discovery of a mobile phone inside an unexploded bag bomb on the wrecked El Pozo train led to the arrest of Moroccan Jamal Zougam. Survivors said Mr Zougam was seen loading bags onto trains at Alcala de Henares.
Explosives in the bag bomb were traced to a mine in northern Spain. Spaniard Jose Emilio Surez Trashorras was charged with supplying some of the explosives to the bombers. A 16-year-old was later jailed for six years for trafficking some of explosives used.
Investigations into a batch of mobile phone cards believed to have been used by the bombers led police to a flat in the Madrid suburb of Leganes in April 2004, where seven key suspects blew themselves up.
ARRESTS: Police arrested more than 70 suspects in an investigation spanning six countries. One suspect, Mohamed Afalah is believed to have died in a suicide attack in Iraq in 2005.
International arrest warrants resulted in the detention and extradition to Spain of a number of the key suspects, who ended up on trial and convicted.