Spanish authorities investigating last month's Madrid train bombings jailed a Moroccan engineering student allegedly linked to the attacks' suspected ringleader and arrested three more suspects, court officials said Monday.

The electrical engineering student, Fouad Almorabit, 28, was jailed Monday on charges of collaborating with a terrorist organization. During hours of questioning by Judge Juan del Olmo, Almorabit admitted knowing suspects in the March 11 attacks that killed 191 people but denied having any role in them, the officials said.

Almorabit was first arrested March 24, released and then arrested two more times, including Thursday, for further questioning. During that first arrest, authorities also seized Almorabit's roommate, Basel Ghayoun, a Syrian now jailed as a prime suspect.

Del Olmo's decision to jail Almorabit raises to 18 the number of people charged in the case. Six are accused of mass murder and 12 of belonging to or collaborating with a terrorist organization.

Fourteen of the 18 people charged in Spain's worst terrorist attack are Moroccan.

The Spanish government says the focus of its probe is the Moroccan Islamic Combatant Group (search), which has links to Al Qaeda (search) and is related to an organization blamed for last year's bombings in Casablanca, Morocco, that killed 45 people, including 12 attackers.

In the Madrid attacks, 10 bombs ripped through four commuter trains during the morning rush hour, also injuring about 1,800 people. Forty-six victims remained hospitalized Monday.

Almorabit, according to officials, had close ties to Serhane Ben Abdelmajid Fakhet, a 35-year-old Tunisian whom the government says led and coordinated the attacks and was among seven terror suspects who blew themselves up April 3 in an apartment south of Madrid as police raided.

Until late last year, Almorabit lived with one of those seven suspects, although it is not known which one. Almorabit's telephone records show that he spoke to suspects in the days before, and on the day of, the coordinated bombings, officials said.

At least six people have been arrested since Friday but only three — all Moroccans — were detained, court officials said. They will appear before the judge Thursday, officials said.

Two of those detainees were arrested Friday in Parla, south of Madrid. They were identified as Hassan Belhadj and Said Aharouch. Belhadj might be related to one of the seven suspects who died April 3, officials said.

The third detainee, identified as Ibrahim Afalah, was arrested Sunday but officials did not say where.

Spanish authorities say the core members of the terror cell believed responsible for the Madrid bombings are either behind bars or dead. At least six more suspects are wanted.

Spain remains on alert after a letter and a video from an Al Qaeda-linked group claiming responsibility for the Madrid attacks warned of more violence unless Spain withdraws its troops from Iraq and Afghanistan. Spain has 1,300 soldiers in Iraq and 125 in Afghanistan.