Mark Manes, 22, admitted selling a TEC-DC9 semi-automatic pistol to Dylan Klebold and Eric Harris for $500 in January.

The prosecution said Klebold used the pistol to kill four people and wound two others. He fired the weapon 55 times during the attack on 20 April.

Klebold and Harris killed 12 students and a teacher at the Denver school before taking their own lives.

It is against the law in Colorado to supply a handgun to a minor. Both were under 18 when they bought the weapon.

Killers thanked Manes

The court heard that Harris had thanked Manes for his help, in a videotape seized by police.

According to transcripts read out by prosecutors, Harris said: "I'd like to make a thank you to Mark and Phil [Duran - accused of introducing the gunmen to Manes].

The infamous pair made several videotapes

"Very cool. You helped us do what we needed to do. Thank you."

But the gunmen also said: "Don't arrest any of our friends, don't arrest any of our family, don't arrest any of our co-workers. They didn't have a [expletive] clue."

Prosecutors acknowledged Manes had known nothing of the plot to attack the school.

But District Judge Henry Nieto said the grave harm resulting from the sale of the gun had to be taken into consideration in sentencing.

He said the shooting was an example of "the worst that could have occurred".

Manes also pleaded guilty to selling the assailants 100 rounds of 9mm ammunition for $25 the night before the attack. He was given an additional three years to be served concurrently for possessing a sawn-off shotgun.

Apology

Before he was sentenced, Manes said he wished he had listened to warnings from his mother to stay away from firearms.

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"I never want to see a gun for the rest of my life," he said.

"I can't adequately express my sorrow to the families of the victims ... I am truly very sorry," he said.

Manes' defence attorney Robert Ransome urged the judge to give his client probation.

"It's not what was put into Harris and Klebold's hands that caused the deaths of those children, it was what was put into their minds," he said.

"None of us knew what Harris and Klebold were up to. None of us," he said.

He also pointed out that the gunmen had "robbed this community of the ability to punish them".

Manes is the first person to be imprisoned in connection with the massacre. Mr Duran's case has not yet come to trial.

'Justice served'

Dozens of friends and relatives of the victims applauded as Manes was escorted out of the courtroom.

Kristen Townsend, sister of shooting victim Lauren Townsend, said: "Did Mark Manes shoot and kill my sister? No ... But he sold the weapons and he must be punished for it."