The 'poisonous' cloud hanging over political life in America

The President called Congresswoman Giffords a "friend" and an "extraordinary public servant".

FBI Director Robert Mueller was due to travel to Arizona yesterday to help co-ordinate the investigation into the shooting which left six people dead, including nine-year-old Christina Greene and Federal Judge John Roll.

Mrs Giffords was shot on Saturday as she met constituents at a Tucson supermarket.

The President said: "It's not surprising that Gabby was doing what she always does – listening to the hopes and concerns of her neighbours. That is the essence of what our democracy is all about.

"That is why this is more than a tragedy for those involved. It is a tragedy for Arizona and a tragedy for our entire country."

In Britain, Shadow Foreign Secretary Yvette Cooper, MP for Castleford and Pontefract, said: "The tragedy in Arizona is truly appalling.

"Congresswoman Giffords and those at this public event were involved in something so normal yet important for any democracy: meeting, talking and debating.

"Our thoughts are with those who have lost loved ones and wishes for a speedy and full recovery are with those who have been injured."

Mrs Giffords is a moderate Democrat who narrowly won re-election in November against a Republican candidate supported by the ultra-conservative Tea

Party movement who sought to throw her from office over her support of the health care law.

Pima County Sheriff Clarence Dupnik, in a pull-no-punches news conference, linked the shooting in Tucson to a poisonous underlying political atmosphere.

"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," said Mr Dupnik, referring to the gunman, identified as 22-year-old Jared Loughner.

Mr Dupnik said Arizona, embroiled over the last year by bitter divisions over illegal immigration and health care reform, has "become the mecca for prejudice and bigotry."

Many officials had been threatened, including Mrs Giffords, the federal judge killed by her side and Mr Dupnik himself.

The sheriff, 74, said: "Pretty soon, we're not going to be able to find reasonable, decent people who are willing to subject themselves to serve in public office."

Mrs Giffords, one of her state's most high-profile Democrats, seems at first glance to be an

unlikely choice of voters in conservative-leaning southern Arizona.

But she has managed to remain popular, winning election three times in the Tucson-area congressional district by holding centre positions, reaching out to constituents and bucking her party's position on many issues.

She has been tough on border security, but supports comprehensive immigration reform.

She voted for Mr Obama's stimulus and health care reforms, but pushed the administration to put armed National Guard troops on Arizona's border with Mexico to stop drug and human smuggling.

Mrs Giffords had been mentioned as a possible Democratic nominee in 2012 for the US Senate seat held by Republican Jon Kyl or for the governor's office in 2014.

A one-time Republican, Mrs Giffords became a Democrat in 2000 and won election to the Arizona house of representatives, where she served one term.

In 2002 she became the youngest woman elected to the Arizona senate, was re-elected in 2004, then stepped down in 2006 to try for the seat of retiring Republican Rep Jim Kolbe.

Mrs Giffords met her husband, astronaut Mark Kelly, who has piloted space shuttles Endeavour and Discovery, in 2003 while they were serving on a committee in China. They married in January 2007.

The couple have no children together. Mr Kelly has two daughters.

Loner with a troubled past

Authorities in Arizona described Jared Loughner as a young man with a troubled past while neighbours spoke of a loner .

One former classmate said Loughner, 22, often did his own thing. Another described him as a student who disrupted class with occasional outbursts.

Neighbours said Loughner was not hostile but did not warm up to anyone either.

"He was a guy in high school who definitely had his opinions on stuff and didn't seem to care what people thought of him," said Grant Wiens, 22, a former classmate at Pima Community College.

Loughner was in custody last after authorities said he opened fire outside a grocery store as Democrat Ms Giffords met voters in Tucson. Authorities said the exact motivation for the shooting was not immediately known.

Investigators were yesterday poring over versions of a MySpace page belonging to Loughner and over a YouTube video published weeks ago under an account "Classitup10" and linked to him.

The MySpace page, which was removed within minutes of the gunman being identified by officials, included a mysterious "Goodbye friends" message published hours before the shooting.