The shooting of a U.S. congresswoman during a political gathering in Arizona has left many questioning the role the increase of vitriolic rhetoric in American politics played in the incident.

Democratic Rep. Gabrielle Giffords was critically injured on Saturday after she was shot in the head during a shooting spree that killed six people attending a public event.

A suspect taken into custody shortly after the attack is said to have harboured anti-government resentment and shown evidence of a scattered mind in online postings. Still, the attack has turned the heat up in Washington, where in recent years politics has reached its possibly ugliest level of partisanship.

Democratic Rep. Raul Grijalva, who represents a neighbouring Arizona district, said the attack may be the work of a "single nut," but that the nation must now consider the result of "an atmosphere where the political discourse is about hate, anger and bitterness."

Lawmakers in Washington were left deeply shaken by the attack. The head of security of the House of Representatives urged members to stay in touch with local law enforcement agencies, although he said in an email that he did not believe further attacks are imminent.

Arizona's Pima County Sheriff Clarence Dupnik said that the level of vitriol in recent political discussion must take some responsibility for the shootings.

"To try to inflame the public on a daily basis, 24 hours a day, seven days a week has (an) impact on people, especially those who are unbalanced personalities to begin with," Dupnik told reporters on Sunday.

Political groups have said that politics has become too personal and nasty, saying this incident should be a call to bring civility back to public debate.

"There has been a lot of discourse about how heated the rhetoric has become in American politics, and many people have suggested that we have reached the apex here," CNN correspondent Paul Vercammen told CTV's Question Period from Tucson.

"So many people seem to be leveling very, very insulting charges against their rivals, whoever they are. There seems to be some reflection going on right now."

The increase in heated rhetoric began during the 2009-2010 debate over U.S. President Barack Obama's health care bill, when critics began suggesting Obama's plan would create government "death panels."

In the wake of the shooting, Republican House leaders have postponed a Wednesday vote to repeal the new health care law.

Gun imagery has been evoked by politicians on all sides of the debate in an apparent bid to raise the political stakes.

Former Alaska governor and vice-presidential candidate Sarah Palin posted a map to her website in March marking some 20 congressional districts, including Giffords', with gun crosshairs and noted that supporters should "reload" and "aim" during November's midterm election.

Giffords expressed concern about the firearm imagery well before the shooting. In an interview after her office was vandalized following the House vote to pass the health-care bill, she referred to the animosity expressed towards her by conservatives, including Palin's decision to list Giffords' seat as one of the top "targets" in the midterm elections.

"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 gun sight over our district. When people do that, they have to realize that there are consequences to that action," Giffords said in an interview with MSNBC.

Giffords, 40, a moderate Democrat, narrowly won re-election in November against Tea Party candidate Jesse Kelly, who sought to turf her from office over her support of the health care law. Anger over her position became violent at times -- not only was her Tucson office vandalized, but someone showed up at a recent gathering with a weapon.

According to the Arizona Daily Star, a photo of Kelly was posted to his website depicting him holding a gun with a headline inviting supporters to a gun-shooting event.

"Get on Target for Victory in November. Help remove Gabrielle Giffords from office. Shoot a fully automatic M16 with Jesse Kelly," the site read, according to the Daily Star.

In the hours after the shooting, Palin issued a statement in which she expressed her "sincere condolences" to the family of Giffords and the other victims. Kelly posted a similar note of mourning on his campaign website.

With files from The Associated Press