by Rochelle van Amber, BBC News, for the BBC's Biodun Iginla

Arizona authorities early Sunday morning were searching for a person of interest in connection with the assassination attempt that left six dead and a congresswoman in critical condition as the nation tried to come to grips with the attack.

The Pima County sheriff's office released a photo of a man they described as a person of interest who was wanted in connection with Saturday's shooting, which left 13 wounded, including Democratic Rep. Gabrielle Giffords, 40. The unknown person was photographed by a surveillance camera near the shooting suspect, Jared Lee Loughner, 22.

The person appears to be white with dark hair and about 40 years old.

"We want to know if the person of interest is associated with" the suspect, Pima County Deputy Jason Ogan said in a telephone interview. "We released the photo to see if anyone knows him."

The suspect was in federal custody Sunday, Ogan said. On the orders of President ObamaFBI Director Robert Mueller traveled to Arizona to head the investigation.

Loughner was apprehended by people in the crowd after the midmorning shooting at a Tuscon supermarket. Witnesses described how he sprayed the area with bullets, killing among others, Arizona's chief federal judge and a 9-year-old girl, who was later identified as the daughter of a Dodgers scout.

Officials wouldn't discuss the motive for the attack, but witnesses said it appeared focused.

"I feel like he knew what he came there to do and he done it," Joe Zamudio said Sunday morning on MSNBC.

Authorities identified the dead as U.S. District Judge John Roll, 63; Christina Greene, 9; Giffords aide Gabe Zimmerman, 30; Dorothy Morris, 76; Dorwin Stoddard, 76; and Phyllis Scheck, 79. Roll had just stopped by to see his friend Giffords after attending Mass.

But officials said they believe that Giffords, who had been sworn in for her third term with the rest of the 112th Congress last week, was the primary target. At 40, Giffords was considered a moderate Democrat who favored immigration reform and who had been the subject of at least two "unfortunate incidents" during the recent campaign for reelection. Giffords defeated "tea party" candidate Jesse Kelly by just 4,000 votes.

In a typically heated congressional campaign in the recent, hyper-partisan midterm elections, Kelly blamed Giffords for supporting President Obama's healthcare overhaul and for her more liberal views on immigration reform, a sore subject in Arizona, a border state whose efforts to halt illegal immigration have drawn condemnation from liberals and the federal government.

On Saturday, Giffords was shot in the head while meeting constituents. She was taken to University Medical Center, where spokeswoman Darcy Slaten said the congresswoman was in critical condition Sunday morning after undergoing two hours of surgery. Hospital officials said they were guardedly optimistic

Slaten said nine other wounded were being treated at the hospital, four of them critical and five serious. Three of the wounded had been treated and released.

While authorities sorted out the details of the shooting, the political world continued to reel as the attack raised fears of a new level of partisan animosity.

In a televised statement from a government building near his home of West Chester, Ohio, House Speaker John Boehner asked that flags at the Capitol be flown at half-staff in honor of the victims. He again condemned violence.

"An attack on one who serves is an attack on all who serve," Boehner said. "Such acts of violence have no place in our society."

Boehner said the House will postpone its scheduled business this week to deal with any needed actions in the wake of the shooting, including if additional security measures were needed.

The House had been scheduled to vote on a repeal of the Obama healthcare program Wednesday. The action was largely symbolic, fulfilling a GOP pledge in the election campaign. But any repeal was expected to fail in the Senate and would face a veto from Obama.

Boehner's comments echoed those by Obama right after the attack.

Praising Giffords as an extraordinary public servant, the president also condemned the shooting.

"We do not yet have all the answers," Obama said Saturday. "What we do know is that such a senseless and terrible act of violence has no place in a free society.