Police said Carla Hochhalter, 48, shot herself in the head with a .38 calibre handgun while a shop assistant was running a background check to authorise her purchase.

Mrs Hochhalter brought ammunition with her into the Alpha Pawn Shop in Englewood, Colorado and asked to see the gun, according to local police. When the assistant turned his back, she apparently loaded the weapon and fired twice, striking herself once and hitting the wall with a second bullet.

She left a note, but its contents were withheld by authorities. Her 17-year-old daughter Anne-Marie was shot several times in the spinal cord, back and chest in the 20 April shootings at the high school in Littleton, Colorado.

Carla Hochhalter: Police said she left a suicide note

She was one of the most seriously injured of the 23 students who were wounded when Eric Harris, 18, and Dylan Klebold, 17, went on the rampage through the school. The pair also killed 12 of their fellow students and a teacher before turning their guns on themselves.

Anne-Marie was left partially paralysed in the shootings. Reports said the 17-year-old had only moved her legs for the first time this week.

Members of the Littleton community - still trying to come to terms with their grief following the massacre - were shocked at the news of Mrs Hochhalter's death.

"I'm absolutely horrified," said Steve Cohn, the father of a 15-year-old student who survived the shootings.

Anne-Marie has only just begun moving her legs again

"We've been under a lot of stress recently but what do you do?"

Jefferson County District Attorney Dave Thomas said: "This pain and suffering is not fair. We do not understand it.

"All we can do is share it with the Hochhalters and others in the community."

Mrs Hochhalter's suicide was the latest blow to the people of Littleton.

On Tuesday, a 17-year-old Columbine student was arrested after allegedly threatening to "finish the job" started by Harris and Klebold.