PARIS—The French Interior Ministry on Saturday said it downgraded a terror alert that had been in effect in Toulouse since a shooter Monday attacked a Jewish school, lowering it a notch from the maximum "scarlet" to "red."

French prosecutors, meanwhile, said they continue to investigate whether Mohamed Merah, who police say claimed responsibility for seven killings—three children and a teacher at the school and three soldiers in other attacks—had any accomplices. Mr. Merah was shot dead by SWAT police on Thursday.

Mr. Merah´s mother, who had been detained since Wednesday when her son barricaded himself in a small Toulouse apartment, was released from police custody, a spokeswoman for the Paris prosecutor said on Saturday. Her lawyer, Jean-Yves Gougnaud, said in an interview on French television that she told him "she saw nothing coming" and felt guilty for what had happened.

Prosecutors said they are now focusing their probe on Mr. Merah´s older brother, Abdelkader Merah, who has also been in police custody since Wednesday and was flown to the headquarters of the antiterrorist police near Paris on Saturday.

French daily Le Parisien said in an article published Saturday citing police sources that Mr. Merah has told investigators that he didn´t know anything in advance about his brother´s alleged attacks on soldiers and the Jewish school.

The older Mr. Merah, however, told investigators that he was "proud" of his brother´s alleged actions, the Le Parisien article said. The older Mr. Merah also said that he was with his brother when a scooter used in the attacks was allegedly stolen.

Prosecutors declined to comment on the Le Parisien article. It wasn´t clear whether Mr. Merah has legal representation.

Mr. Merah´s older brother had been on the radar of French antiterrorist agencies because he allegedly helped Islamic militants travel illegally from Europe to Iraq in 2007, police officials have said. The brother was never prosecuted or charged, they said. Prosecutors said this week that what focused police attention on the older Mr. Merah was that around that time he had sent a small amount of money to an inmate convicted for being part of a jihad network.

French prosecutors have characterized the older Mr. Merah as a Salafist, referring to a sect that has a radical practice of Islam but isn´t known for being violent.

In a parallel effort, French investigators say they are also trying to firm up their conviction that the suspected killer had no connection to al Qaeda and did not train with Taliban groups, though he travelled to Afghanistan in 2010, and to Pakistan in 2011.

During the 33-hour-long siege of his Toulouse apartment that ended on Thursday, Mr. Merah, according to the SWAT police, said that he belonged to al Qaeda and had been trained by the organization.

France and the U.S. have said that they have no evidence to support this claim.

Mr. Merah was never a member of Islamic Emirate of Afghanistan, Taliban spokesman Zabihullah Mujahid said on Saturday, referring to the name that the Taliban leadership uses to describe itself.

Pakistan authorities have said they had found no trace of Mr. Merah in their files. "He´s not in our databank nor was he ever on our radar," an officer with Pakistan´s Inter-Services Intelligence Directorate military spy agency said on Friday.

—Habib Khan Totakhil contributed to this article. (Copyright © Dow Jones & Company, Inc.) 03/24/12)