CHICAGO -- An American was sentenced on Thursday to 35 years in prison for the key role he played in a 2008 terrorist attack on Mumbai that has been called India's 9/11, leaving open the possibility that he one day could go free. Victims called the sentence unjust for the severity of the violence.

David Coleman Headley's meticulous scouting missions facilitated the assault by 10 gunmen from a Pakistani-based militant group, which killed 160 people -- including children.

The attackers arrived by boat on Nov. 26, 2008, carrying grenades and automatic weapons, and fanned out to hit multiple targets, including a crowded train station, a Jewish centre and the landmark Taj Mahal Hotel. TV cameras captured much of the three-day rampage live.

The attack heightened the strain in a historically antagonistic relationship between India and Pakistan, which have fought three major wars. Indian officials accuse Pakistani intelligence of helping to plan the assault -- an allegation Pakistan denies.

The maximum sentence Headley, 52, faced was life in prison. He agreed to co-operate with U.S. authorities and plead guilty in 2010 to 12 counts to avoid what would have been his maximum sentence: death. He also secured a promise not to be extradited to India.

U.S. District Judge Harry Leinenweber said he considered Headley's co-operation with federal prosecutors in imposing a sentence within the range that prosecutors had requested, even though "the damage that was done was unfathomable."

The judge added, "I don't have any faith in Mr. Headley when he says he's a changed person and believes in the American way of life."

Headley, who did not address the court, showed no emotion when the sentence was announced.

Late year, India secretly hanged the lone surviving gunman, Mohammed Ajmal Kasab.

Citing what they described as valuable intelligence Headley provided authorities about terrorist networks since his arrest, prosecutors had asked for a relatively lenient sentence of between 30 and 35 years.

The charges included conspiracy to aid the Pakistani-based group, Lashkar-e-Taiba, that mounted the attacks, as well as conspiracy to commit murder in India and aiding and abetting in the murder of six Americans.

Headley, a small-time drug dealer-turned-terrorist plotter, seemed to leap at the chance to spill secrets following his 2009 arrest and continued providing details even after the U.S. government agreed not to seek the death penalty in exchange for his co-operation.

He never pulled a trigger in the attack, but he conducted meticulous scouting missions -- videotaping and mapping targets -- so the attackers who had never been to Mumbai adeptly found their way around.

"What he did was unfathomable," said James Kreindler, an attorney for relatives of U.S. victims. "Imagine what is going through a person's mind who is videotaping these places knowing what will happen there later."

In big cases where suspects co-operate, prosecutors often ask for leniency. It's both a reward and a message to future suspects that they, too, could get a break if they spill their secrets. Still, for a reviled figure like Headley to get a sentence less than sentences routinely given to convicted drug traffickers or child pornographers could prompt criticism.

Prosecutors seemed to anticipate that in their filing, acknowledging that, "Determining the appropriate sentence for David Headley requires consideration of uniquely aggravating and uniquely mitigating factors."

Prosecutors have recounted only in broad terms how Headley has shed light on the leadership, structure and possible targets of Lashkar-e-Taiba, which was believed to have ties to the Pakistani intelligence agency, known as ISI. Headley has said his ISI contact was a "Major Iqbal," who was named in the indictment that charged Headley.

Seth Jones, a RAND Corp. political scientist, agreed that Headley must have provided useful insight for U.S. intelligence, especially about how Pakistani intelligence agents allegedly reach out to people like Headley.

"From my perspective, this was pretty detailed information about one ISI contact (Headley) with one handler, Iqbal," Jones said. But he added Pakistani intelligence would have been careful not to reveal too much to Headley, saying, "They didn't trust him either."

Survivor Andreina Varagona described in a presentencing filing dining with the Scherrs at the hotel restaurant when gunmen burst in. Bullets tore apart the room as they dove under a table, the girl screaming.

"I suddenly felt the warm spray of blood on my face and in my hair. ... Naomi's screams had stopped too, and I saw her lying lifeless beside (her father)," she recounted. "They'd both been shot dead."