A woman pauses Wednesday at a makeshift memorial outside the University Medical Center in Tucson, Ariz., for the victims of the Jan. 8 shooting, which left six dead and 14 wounded. (Mike Segar/Reuters)

U.S. President Barack Obama says the mass shooting that left six people dead and 14 injured should not be blamed on a lack of civility and cautioned against pointing fingers and assigning blame.

"Bad things happen and we must guard against simple explanations in the aftermath," Obama said Wednesday night at a memorial in Tucson, Ariz.

The ceremony at the University of Arizona gymnasium was in honour of the victims shot on Saturday at a community outreach event hosted by congresswoman Gabrielle Giffords at a Tucson grocery story.

Obama said Giffords, who survived being shot in the head, opened her eyes for the first time shortly after he visited her at the University Medical Center before the memorial service.

"She knows we're here, and she knows we love her and she knows we are rooting for her," Obama said.

The assassination attempt on Giffords has sparked debate over U.S political discourse and the role it may have played in motivating the gunman. But Obama said no one can know exactly what triggered the attack or what might have prevented it.

The tragedy cannot be used to turn against one another, he said.

"Rather than pointing fingers or assigning blame, let us use the occasion to expand our moral imaginations, to listen to each other more carefully, to sharpen our instincts for empathy, and remind ourselves of all the ways our hopes and dreams are bound together," Obama said.

U.S. President Barack Obama hugs congresswoman Gabrielle Giffords's husband, NASA shuttle commander Mark Kelly, at a memorial event in Tucson. (Jim Young/Reuters)

Obama said if the tragedy prompts debate, it should be "worthy of those we have lost."

"Let's make sure it's not on the usual plane of politics and point-scoring and pettiness that drifts away with the next news cycle."

If the deaths help usher in more civility in public debate, "let us remember it is not because a simple lack of civility caused this tragedy — it did not — but rather because only a more civil and honest public discourse can help us face up to our challenges as a nation."

Earlier, a doctor said Giffords is now less sedated and more responsive than she had been.

Dr. Peter Rhee, the trauma chief at the hospital, said Giffords's condition remains stable and she has not had any setbacks.

Suspect was stopped

More details also emerged Wednesday of the alleged gunman's actions on the day of the shooting. Authorities said Jared Loughner, 22, was pulled over for running a red light less than three hours before the shooting.

Michelle Andrews of Tucson listens with an overflow crowd as Obama speaks at the memorial service Wednesday at the University of Arizona. (Charlie Riedel/Associated Press)

A wildlife officer with the Arizona Game and Fish Department pulled Loughner over and took his driver's licence and vehicle registration information. The officer let Loughner go after finding no outstanding warrants for him or his vehicle.

"He had a valid licence, the car was registered, he had insurance," department spokesman Jim Paxon said.

"He was warned and released because we had no probable cause to hold, or do an extensive search," he said.

Loughner is being held without bail in a Phoenix jail. He is charged with one count of attempted assassination of a member of Congress, two counts of killing an employee of the federal government and two counts of attempting to kill a federal employee. State charges have not been filed yet.

With files from The Associated Press