A gunman opened fire outside a supermarket in Tucson, Arizona, on Saturday, killing six and wounding 12, including a US congresswoman, CNN informed.

US Democratic Representative Gabrielle Giffords, aged 40, was hospitalized Saturday in critical condition. She underwent a surgery for a single gunshot wound to the head. Doctors said they were "very optimistic about her recovery."

Among the dead are a federal judge and a 9-year-old girl, local authorities told CNN.

One man, aged 22, was arrested immediately after the shooting and a second man, believed to have been involved in the attempted assassination, is being searched.

The attack happened about 10 minutes after the start of a previously scheduled public meeting outside a grocery store.

The local Sherriff, Clarence Dupnik, has announced that the shooter moved towards the crowd and began firing. It was unclear if he said anything or who was shot first, he said at a news conference.

According to an employee of a nearby business, there were between 15 to 20 shots. Two people tackled the gunman, who still had ammunition remaining, the sheriff said.

"He probably would have shot other people had he not been tackled," Dupnik said.

Five people died at the scene. The sixth victim, a 9-year-old girl, died at the hospital.

Dupnik said that the suspect had a criminal past, in which he made violent threats.

"All I can tell you is there's reason to believe this individual may have a mental issue," he said.

The press secretary of the US Representative said he was unaware of any recent threats against Giffords.

Dupnik said two incidents occurred in 2010 involving Giffords, including one in which a gun fell out of a constituent's pocket during an angry town hall meeting and another in which windows at her campaign office were broken out.

The US President Barack Obama called the attack "unspeakable tragedy."

"We do not yet have all the answers. What we do know is that such a senseless and terrible act of violence has no place in a free society," he said.