A gunman in Arizona who killed six people and injured 14 others, including a congresswoman, could have claimed more victims had a woman not tried to seize his weapon, police said Sunday.

Democrat Gabrielle Giffords with her husband, Capt. Mark E. Kelly, a NASA astronaut and navy pilot from New Jersey. (Reuters)

Jared Lee Loughner, 22, was arrested in the shooting. The U.S. government on Sunday filed criminal charges, including attempted assassination, against him in a court in Phoenix.

The suspect was reloading his semi-automatic Glock when 61-year-old Patricia Maisch — who was in the crowd at the outdoor political event in Tucson — wrestled for control of the handgun, Pima County Sheriff Clarence Dupnik said Sunday.

"He inserted the new [31-bullet] magazine but it didn't fire," the sheriff told reporters, adding the woman was "wounded when she did this." Dupnik did not know the extent of her injuries.

After the skirmish, two men, Roger Sulzgeber and Joseph Zimudie, stepped in and tackled the gunman, wrestling him to the ground.

"There would have been a greater catastrophe had he been successful in [inserting the magazine properly]. Fortunately, the spring and the magazine failed and the two gentlemen were able to get it away from him and subdue him," Dupnik said.

Still, the gunman succeeded in shooting several people who had come out for a publicized meet-and-greet with Gabrielle Giffords outside a Safeway grocery store. The 40-year-old Democrat was left in critical condition after a bullet traversed one side of her brain from back to front.

Emergency personnel attend to a shooting victim outside a shopping centre in Tucson after Saturday's shooting. (James Palka/Associated Press)

Giffords remained the most seriously wounded of the victims still in hospital on Sunday. But doctors said she was able to open her eyes and answer simple questions — non-verbally, because she was on a ventilator.

The shooting won't stop members of the U.S. House of Representatives from carrying out their duties, U.S. House Speaker John Boehner said a day after the attack.

In a brief statement Sunday morning, the newly sworn speaker said flags on the House side of the Capitol in Washington will be flown at half-mast to honour Giffords's slain aide, Gabe Zimmerman. Others killed included a federal judge, a nine-year-old girl, and three senior citizens.

Boehner said normal House business this week is postponed to focus on any necessary actions in the shooting aftermath. But he said the "inhuman act" can't be allowed to deter representatives from serving their constituents.

The Republican speaker didn't take any questions before leaving the township government building near his West Chester, Ohio, home.

Lawrence Cannon, Canada's foreign affairs minister, offered his condolences to the victims' families and called the shooting a "senseless act of violence."

"In particular, I offer sympathy to the family of the young girl among the victims, whose entire life was ahead of her," he said in a statement released late Saturday.

On Saturday, police described Loughner as mentally unstable and the author of previous death threats. He reportedly left a trail of rambling internet postings that have since been removed from the web.

A photo of shooting suspect Jared L. Loughner from the 2006 Mountain View High School yearbook. (Associated Press)

His name was linked to some postings in which the author accused the U.S. government of mind control and demanded a new currency.

Dupnik said there had been earlier contact between Loughner and law enforcement after he had made death threats. But Dupnik said the suspect had not threatened Giffords. It is not clear why she was targeted.

Giffords had started her third term in Congress last November after defeating a Tea Party-backed Republican.

This graphic from Sarah Palin's Take Back The 20 Facebook page shows a U.S. map with the crosshairs of a gun scope imposed over 20 Democrats' districts. The graphic no longer appears on the webpage. (Associated Press)

Former Alaska governor Sarah Palin last year listed Giffords's seat as one of the top "targets" in the November midterm elections because of the lawmaker's support for the health-care bill.

"For example, we're on Sarah Palin's targeted list, but the thing is, that the way that she has it depicted has the crosshairs of a gunsight over our district. When people do that, they have to realize that there are consequences to that action," Giffords had said in an interview with MSNBC.

The map identifying the districts was removed from Palin's "Take Back The 20" Facebook page on Saturday.

Loughner was taken into custody soon after the shooting. Shortly after the arrest, police said they were seeking a possible accomplice, a white male in his 50s.

However, Arizona authorities said on Sunday the man they called a "person of interest" was later cleared of any involvement in the attack.

Pima County Sheriff's Deputy Jason Ogan said the man was a cab driver who drove the gunman to the Safeway store.

Ogan said the man contacted police after his photograph, captured on security cameras, was distributed to the media.

With files from The Associated Press