A US congresswoman has been shot in the head and six other people have been killed by a gunman in Arizona.

美国亚利桑那州一位国会女议员被一位歹徒击中头部，另有六人被射杀。

Democratic Representative Gabrielle Giffords, 40, was shot at close range（接近地） during a public meeting in Tucson.

She is in a critical condition, but the doctor treating her said he was "very optimistic about her recovery".

The dead included a nine-year-old girl and a federal judge. President Barack Obama said the shooting was a "tragedy for our entire country".

The Associated Press news agency reported that one of Ms Giffords' political aides was also killed in the shooting.

In addition to the six deaths, police said a total of 13 people - including Ms Giffords - had been shot and wounded in the attack.

A suspect named by US media as Arizona resident Jared Loughner, 22, was arrested after the incident.

Local police, who have not confirmed the suspect's name, said they were hunting a possible accomplice¹（同谋者，共犯） .

"[The suspect] has kind of a troubled past, and we're not convinced that he acted alone," said Pima County Sheriff Clarence Dupnik.

"There is some reason to believe that he came to this location with another individual."

Presidential tribute

The attacker struck as Ms Giffords held one of her regular open-invitation meetings - which she called "Congress on your corner" - with her constituents²（成分，分子） outside a supermarket in Tucson.

An eyewitness³ to the killings⁴, Steven Rayle, told AP he had played dead to escape.

"I had passed by the table, the congresswoman was standing⁵ there talking to several people, I went to the side of the table... and I looked up and I saw a man shoot her in the head," said Mr Rayle.

"Then he began just spraying gunfire everywhere. At that point I ducked behind the concrete post.

"The whole thing unfolded maybe 12 or 15 seconds. As he came around [the post], I lay on the ground and acted as if I were shot."

The gunman was reportedly overpowered by members of the crowd, before being taken away by police.

The dead judge was named by officials as John Roll, who had spent more than 40 year on the federal bench. It is not thought that he was deliberately⁶ targeted.

Mr Obama urged the nation to "come together, and support each other".

"I ask all Americans to join me and Michelle in keeping all the victims and their families, including Gabby, in our thoughts and prayers," he said.

He described Ms Giffords as a personal friend and an "extraordinary public servant".

Mr Obama has tasked FBI director Robert Mueller with overseeing the investigation⁷.