Said Kouachi, one of the two brothers being sought by French authorities in the massacre at Charlie Hebdo magazine, traveled to Yemen in 2011 to be trained by al-Qaeda in the Arabian Peninsula, two senior U.S. counterterrorism officials told NBC News on Thursday. The training lasted for several months, one of the officials said.

No other details were immediately available, but the disclosure added to the evidence that Kouachi, 34, and his brother Cherif, 32, had been on counterterrorism agencies' radar for several years before the attack Wednesday that killed 12 people at the Paris-based satirical weekly. A Homeland Security official told NBC News separately on Thursday that the brothers had been in the U.S. terrorism database and on the U.S. no-fly list "for years."

Cherif Kouachi, in fact, was sentenced to prison in 2008 after a Paris court found him and six other men guilty of helping funnel fighters to Iraq, raising the question of why the Kouachi brothers escaped the attention of authorities before the attack. French Interior Minister Bernard Cazeneuve told reporters that both were "being watched over, but there were no elements at the time to warrant starting an inquiry."

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Cherif Kouachi's trial in 2008 began after years of investigation. Authorities said at the time that a militant network had funneled about a dozen French fighters to camps linked to Abu Musab al-Zarqawi, the head of al Qaeda in Iraq, and sought to send more before Zarqawi was killed in a U.S. airstrike in 2006.

Kouachi was arrested days before he planned to travel to Syria, allegedly to be trained. The interior minister told reporters that Kouachi's accomplice at the time had described him as wildly anti-Semitic.

Authorities linked the operation to the 19th Arrondissement Network, named for the Paris district where it was based, which is home to many Muslim families with roots in France's former North African colonies. Kouachi was sentenced to three years in prison, 18 months of which were suspended.

During the trial, Kouachi testified that the alleged ringleader, Farid Benyettou, then a 26-year-old preacher, taught him that suicide bombers could die as martyrs. He also said the abuse of prisoners by U.S. personnel at the Abu Ghraib prison in Iraq had made an impact on him. "I really believed in the idea," he was quoted as saying.

Caseneuve said Kouachi was also indicted in 2010 in the attempted escape of Smain Ait Ali Belkacem, a member of the militant Armed Islamic Group and one of the leaders of a wave of bombings in France in 1995, including a commuter rail bombing in Paris, which killed eight people and injured 117 others. But the case against Kouachi was dismissed.

Cazeneuve said Said Kouachi lived in Reims, where he was unemployed, and had never been charged with a crime but had been connected to some of his brother's court cases.

Less is known about Hamid Mourad, 18, who surrendered at a police station Wednesday night, including his relationship to the brothers.

IN-DEPTH

M. Alex Johnson, Cassandra Vinograd and Erin McClam of NBC News contributed to this report.