The US authorities are understood to be exploring whether the shooting of a Jewish congresswoman in Arizona was motivated by the gunman’s links to an extremist group.

The US Department of Homeland Security (DHS) has noted both that the Congresswoman, Gabrielle Giffords, is Jewish and that her shooter may have been involved with an allegedly antisemitic group.

Ms Giffords, Arizona’s first Jewish representative and a former member of the regional board of the Anti-Defamation League (ADL), is still in hospital in intensive care following the rampage on Saturday at a political event.

A member of her staff, Gabe Zimmerman, and five other people including a nine-year-old girl were left dead after the attack.

The man believed to be behind the attack, Jared Loughner, is said have referred to the radical anti-immigrant organisation American Renaissance in his online activity.

He is also thought to have claimed Mein Kampf by Adolf Hitler as his favourite book.

The DHS document, obtained by US news channel Fox, recorded that the American Renaissance ideology is “anti-government, anti-immigration, anti ZOG (Zionist Occupational Government), antisemitic.”

Mr Loughner, who was arrested on a drugs offence in 2007, had an application rejected from the US army two years ago. He is being held by the FBI and will appear in court today, although investigators are also looking at the possibility that there was a second person involved in the attack.

The chair of the Arizona regional board of the ADL, Miriam Weisman, and its director Bill Straus, expressed their shock over the “unconscionable and horrific act of violence against one of our highly respected public servants”.

They said: “It is critical to determine whether the alleged shooter acted alone or with others, and whether he was influenced by extremist literature, propaganda or hate speech.

“While it is still not clear whether the attack was motivated by political ideology, the tragedy has already led to ‘soul searching’ about the connection between incivility and violence.”

Third-term congresswoman Ms Giffords has been threatened and her office vandalised for her support of healthcare reform legislation and her opposition to tightening immigration controls.

The National Jewish Democratic Council warned that this could also be a motive. They said “The levels of vitriol in our political discourse…have surely contributed to the atmosphere in which this event transpired."