RIO DE JANEIRO - Rio police released a video Wednesday which suggests that the man, who killed 12 children in a school last week, had planned the massacre for months.

The video was in an old hard disk found in the gunman's house. In the recording, 23-year-old Wellington Oliveira said he would avenge those who, like him, had been "humiliated, bullied, and disrespected."

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"Most people who disrespect me, who think I am an idiot, who take advantage of my goodness, who judge me, will find out who I am in the most radical manner," he said in the video.

According to the head of Rio's technical police, Sergio Henriques, the hard disk had not been accessed since July 2010. It was separated from Oliveira's computer, which he had burned before the massacre.

This was not the first video found by the police. In another recording made some days before the crime, Oliveira claimed his actions were against cruel people who prey on the innocent.

"The fight for which I will die is not only against what is known as bullying, but against cruel and cowardly people who take advantage of the goodness, innocence, and weakness of those incapable of defending themselves," he said.

Oliveira entered his old elementary school, in the neighborhood of Realengo, on Thursday morning, under the pretext of giving a speech. Armed with two revolvers and a lot of ammunition, and protected by a bulletproof vest, he shot dead 10 girls and two boys aged 12 to 14, and injured another 11 children.

According to the survivors, he mainly aimed at girls, shooting them in the head, while hitting boys mostly in non-vital areas such as the arms and legs. He was reportedly unmoved by the children who begged for their lives - to stop children from running away, he'd shoot them in the legs and then order them to turn their heads to the wall to be executed.

Oliveira was trying to reach the school's upper floor when he was confronted by a policeman, who injured him. The gunman then killed himself with a shot in the head.