Shooting suspect Jared Loughner is not mentally competent to stand trial on charges he killed six people and wounded US Congresswoman Gabrielle Giffords in January.

US District Judge Larry Burns found Mr Loughner, aged 22, described by his lawyers as ‘gravely mentally ill’, was incapable of understanding the proceedings against him and assisting in his own defence.

The decision came shortly after Mr Loughner was hustled out of the courtroom for an outburst that disrupted the hearing.

He spent five weeks undergoing psychiatric evaluations at a federal prison hospital in Missouri.