The investigation into the Madrid terror attacks is at an early stage but Moroccan Jamal Zougam, 30, is widely reported to be a leading suspect.
Mr Zougam has not been charged with anything and much of the information about him is emerging from Spanish newspaper reports rather than official police information.
Mr Zougam grew up in the Moroccan port city of Tangiers
Spanish media say passengers who survived the attacks told police they saw Mr Zougam leaning against a carriage of one of the trains bombed on 11 March.
Mr Zougam, who ran a mobile phone shop in Madrid, was arrested two days later, along with two other Moroccans and two Indian nationals.
He is said to have been under Spanish police surveillance since the bombings in Casablanca last May that killed 45 people.
Moroccan police believe he has links with the Salafia Jihadia group blamed for those attacks.
Spanish newspaper El Pais said Mr Zougam is thought to have shared a house with Abdelaziz Beyaich, who is in custody in Spain in connection with the Casablanca attacks.
Mr Zougam is also alleged to be a follower of Abu Dahdah, the suspected leader of an al-Qaeda cell in Madrid.
Abu Dahdah has been indicted by Spanish judge Baltasar Garzon on charges of helping to prepare the 11 September 2001 attacks on the United States.
Zougam is alleged to have links with Abu Dahdah (pictured)
Mr Zougam was named in the indictment but not charged.
Police searched his flat in Madrid in November 2001, finding videos from radical Islamist groups and telephone numbers from alleged members of Madrid's al-Qaeda cell.
In addition, the name of a Jamal Zougam appears in court documents outlining the case against a French Muslim convert David Courtailler who went on trial on Wednesday in Paris accused of criminal association with a terrorist group.
It is not clear whether it is the same Jamal Zougam.
Mr Zougam was born in the Moroccan port city of Tangiers on 5 October 1973.
A friend in Madrid described him as a "modern Muslim" and by no means a fanatic.
"He was good looking, he didn't have a beard...He joked around a lot. He was religious; we're all religious," a 28-year-old man called Abdul told Reuters.
"He's the kind of guy who would walk around in a Lacoste t-shirt in summer. He was very modern but he didn't drink alcohol."