TUCSON — A federal judge ruled Wednesday that Jared L. Loughner was not mentally competent to stand trial on charges that he opened fire at a constituent event for Representative Gabrielle Giffords in January, killing six and injuring her and 12 others.

The ruling by Judge Larry A. Burns of Federal District Court suspends the court proceedings against Mr. Loughner while the suspect, who experts said has schizophrenia, undergoes treatment at a federal psychiatric facility in Springfield, Mo.

Before the judge’s decision, Mr. Loughner disrupted the hearing with an outburst that prompted deputy marshals to carry him from the room. He will return to court on Sept. 21, when the judge will review again whether Mr. Loughner understands the 49 charges against him and can assist in his defense.

It remains possible, legal experts said, that Mr. Loughner will never be found competent, and could remain in a psychiatric facility indefinitely, but prosecutors said they hoped the trial would resume. “Our goal has always been and always will be to go to trial,” said Dennis K. Burke, a United States attorney, speaking to reporters outside the courthouse.