PHOENIX (Reuters) - A federal judge on Monday ordered Jared Lee Loughner, the college dropout charged in January's Arizona shooting rampage, to undergo a mental evaluation in Missouri to see if he is competent to stand trial.

Federal prosecutors earlier this month asked for Loughner to receive an evaluation.

U.S. District Judge Larry Burns wrote in his ruling that the purpose of the examination is not to focus on Loughner's mental state at the time of the shooting.

"The question at issue is whether the defendant is presently suffering from a mental disease or defect rendering him mentally incompetent to the extent that he is unable to understand the nature and consequences of the proceedings against him, or to assist properly in his defense," he said.

Loughner's mental competency has been an issue in the case since soon after the January 8 shootings. Officials at Pima Community College in Tucson said he withdrew from the school last year amid questions about his mental health.

Analysts and commentators also cited strange writings and videos that he posted online before the shooting.

Loughner faces federal charges that he opened fire on U.S. Representative Gabrielle Giffords and a crowd of her constituents at a gathering at a Tucson, Arizona, supermarket.

Six bystanders, including a 9 year-old girl and a federal judge, were killed and 13 others, including Giffords, were wounded in the shooting.

In his ruling, Burns agreed with prosecutors that a medical referral center at a federal prison facility in Springfield, Missouri, was the best place for Loughner to be evaluated.

The facility has medical staff that can conduct neurological tests and has psychiatrists available for assistance around the clock, Burns said.

Loughner's attorneys had asked for him to be evaluated in Arizona, arguing that transferring him from Tucson could worsen his mental state and disrupt their relationship with him.

Burns ruled that Loughner's attorneys may retain an independent expert to evaluate him, but that the court-ordered assessment should take place at the Missouri facility.

Burns said he expects a report on the mental health examiners' findings by May 11.

(Writing by Alex Dobuzinskis: Editing by Peter Bohan)

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