WASHINGTON // Police in Tucson, Arizona, are hunting for a second man who may have been involved in the shooting on Saturday in which six people died and 12 were wounded, including a congresswoman who remains in critical condition in hospital.
Law-enforcement officials said they don't believe the man who has been arrested for the shooting, Jared Lee Loughner, 22, a Tucson native, acted alone and released a photograph of a second "person of interest", a 40 to 50-year-old white male.
Barack Obama, the US president, has authorised Robert Mueller, director of the Federal Bureau of Investigation, to take personal control of the investigation, which involves law agencies nationwide.
At a press conference yesterday, Mr Mueller said formal charges would be brought against Loughner. He said law enforcement agencies had "no information at this time to suggest any specific threat remains".
Nevertheless, in Washington, the US Capitol Police said lawmakers should take "reasonable and prudent precautions regarding their personal safety and security".
Clarence Dupnik, the Arizona sheriff, said during a press conference on Saturday night that an atmosphere of "bigotry" may well have played a role in encouraging the suspected gunman.
Describing the man held for the shooting as someone who might have a "mental issue", a clearly emotional Mr Dupnik said "unbalanced" people are "especially susceptible to bigotry".
"When you look at unbalanced people, how they respond to the vitriol that comes out of certain mouths about tearing down the government, the anger, the hatred, the bigotry that goes on in this country is getting to be outrageous. And unfortunately, Arizona, I think, has become sort of the capital."
Arizona has been the scene of perhaps some of the most divisive political rhetoric in recent years in the US. The state, bordering Mexico, has come under criticism from the White House for its immigration laws, for which it faces a lawsuit by the Department of Justice. One of the victims of Saturday's shooting, John Rolf, a federal judge, received hundreds of threats in 2009 after allowing a lawsuit by illegal immigrants against an Arizona rancher to go forward.
Gabrielle Giffords, the Democratic congresswoman who investigators say was the main target for the shooter and who was shot in the head at point-blank range, was an outspoken critic of Arizona's immigration laws.
She had also come under fierce criticism for her support for the Democrats' healthcare reform bill, deeply unpopular in the largely Republican state.
Last March, after the final approval of the healthcare law, the windows of Ms Giffords' office in Tucson were broken in an act of vandalism.
In spite of this, Ms Giffords clung on to her seat in last November's mid-term elections. Indeed, Ms Giffords had been speaking on Saturday outside a Safeway supermarket north-west of Tucson in what was the first opportunity for constituents to meet her since she was sworn in for a third term last Wednesday.
It was there that witnesses said Loughner leapt forward and began firing with what was yesterday revealed to be a Glock nine-millimetre. It is not clear whether he tried to escape or stopped to reload, but he was eventually tackled by two bystanders and taken into custody by the police. Loughner had recently been responsible for bizarre internet postings - at least one showing a gun - and a series of videos in which he made rambling statements on topics such as the gold standard and mind control.
Loughner is not speaking to the police. But speculation about his motives has already filtered from online blogs to the mainstream media. Many have noted that Ms Giffords found herself on a target map released in March last year by Sarah Palin, the former governor of Alaska and Republican vice-presidential candidate in the 2008 elections, in the run-up to this past autumn's mid-term elections.
Mrs Palin's target map featured a list of Democratic seats that she urged Republicans to focus on in the mid-term elections. The list showed a map of the US replete with graphics of gunsight crosshairs.
At the time, the map caused a lot of consternation. Ms Giffords herself spoke about her concerns last year.
"We're on Sarah Palin's target list, but the thing is the way she has got it depicted has the crosshairs of a gunsight over our district," she said. "And when people do that they've got to realise there are consequences to those actions."
Mrs Palin, in a statement on Saturday, offered her "sincere condolences" to the Giffords and the other victims.
"On behalf of Todd and my family, we all pray for the victims and their families, and for peace and justice," she said on her Facebook page.
The target map has also been removed from Mrs Palin's website.
The shooting will re-ignite the debate over gun control in the US. Arizona has one of the country's laxest gun control laws and Ms Gifford had advocated more stringent laws regulating their purchase and banning semi-automatic guns altogether. On Mrs Palin's Facebook page, fans were anticipating this, with one visitor, addressing Mrs Palin, who is staunchly opposed to gun control, directly.
"The anti-gun activists will blame us responsible gun owners and you as our role-model as the cause of this tragedy. This is another lesson that guns don't kill people, people kill people."
Meanwhile, tributes and condemnation have poured in from across the political divide. Mr Obama called Ms Giffords a "friend", during a news conference on Saturday and vowed to get to "the bottom of this".
The new Republican House speaker, John Boehner, said he was "horrified", and declared that, "an attack on one who serves is an attack on all who serve".
John McCain, the Republican senator for Arizona who had chosen Mrs Palin as his running mate in 2008, denounced the person responsible as "a wicked person who has no sense of justice or compassion… whoever did this, whatever their reason, they are a disgrace to Arizona, this country and the human race".
Ms Giffords was yesterday reported to be in an induced coma. In addition to her, five people remain in critical condition, and five are described as in serious condition. Among the fatalities were three constituents, a member of Ms Giffords's staff and a nine-year-old girl.