Video | FBI hunt for second suspect in Arizona massacre

He was pictured at the scene of the attack in Tucson, Arizona, as officers appealed for the public's help to try to identify him.

The suspect is described as white, about 40-50 years old, with dark hair and was last seen wearing blue jeans and a dark blue jacket.

Another man, who is thought to have carried out the shooting, is already in custody after being arrested at the site of the atrocity.

He has reportedly been identified by a federal law official as 22-year-old Jared Lee Loughner.

Police said the gunman opened fire at point-blank range using a semi-automatic pistol.

Among those who were attacked was US congresswoman Gabrielle Giffords, who is in intensive care after being shot in the head.

The shootings took place outside a supermarket where the 40-year-old Democrat was meeting constituents.

The dead included federal judge John Roll, 63, and nine-year-old Christina Greene.

The other people who lost their lives were Phyllis Schneck, 79, Dorothy Murray, 76, Dorwin Stoddard, 76, and 30-year-old Gabriel Zimmerman.

A Tucson fire official said the number of wounded was 13.

Pima County Sheriff Clarence Dupnik said the detained suspect, who was tackled after the shooting and is in custody, has "a troubled past".

Giffords was airlifted to a hospital in Tucson, where she underwent surgery. One of the doctors who treated her said he was optimistic about her recovery.

The bullet travelled straight through her brain, he told reporters.

US President Barack Obama said: "We don't yet know what provoked this unspeakable act," calling the shooting a "senseless and terrible act of violence".

Sky's US correspondent Greg Milam said Obama has dispatched FBI director Robert Mueller to Arizona to oversee the investigation.

Gun violence is common in America, which is periodically rocked by mass shootings, but political shootings are rare.

The shootings followed contentious congressional elections in November.

The polls were marked by heated rhetoric over issues like the Democrat-led drive to overhaul the US healthcare system and immigration reform.

A window in Ms Giffords' office was smashed in March, after Congress passed the healthcare overhaul that had been opposed by Republicans.

"The rhetoric is really heated. Not just the calls but the e-mails, the slurs," Giffords told television network MSNBC at the time.

Giffords was hosting a "Congress on Your Corner" event when the gunman attacked from about 4ft away, National Public Radio said.

The suspect approached the politician from behind, firing at least 20 shots at her and others in the crowd, MSNBC reported.

In March, Republican Tea Party favourite Palin released a 'target list' of Democrats she wanted to see unseated in last November's mid-term elections. Giffords was on the list.

At the time, Palin drew criticism for telling her followers "Don't retreat - instead, reload!"

Regarded as a rising star in the Democratic Party, Giffords was one of the few Democrats to survive the Republican sweep in "swing" districts in the elections.

Giffords is a moderate who narrowly beat a Tea Party candidate who tried to throw her out of office over her support for the health care law.