Rep. Gabrielle Giffords (D-AZ) remained in critical condition Sunday, a day after she was seriously wounded in a shooting rampage that killed at least six people and injured at least a dozen others in Tucson. Police and investigators were trying to piece together the alleged shooter’s motive as they searched for a possible accomplice.

Giffords, 40, was battling for her life hours after the gunman opened fire at a grocery store parking lot, where the congresswoman was meeting with constituents. U.S. District Judge John M. Roll was among the dead.

The three-term Democrat was shot once in the head with the bullet going “through and through,” according to a trauma surgeon at the Tucson hospital where she was airlifted for surgery. The gunman was reportedly carrying a semi-automatic weapon.

Doctors were optimistic about Giffords’ survival as she was responding to commands from doctors.

“With guarded optimism, I hope she will survive, but this is a very devastating wound,” said Dr. Richard Carmona, the former surgeon general who lives in Tucson.

On Sunday morning, University Medical Center spokeswoman Darcy Slaten said Giffords remained in critical condition and that she was sedated after undergoing two hours of surgery.

She said nine other wounded were being treated at the hospital, four of them critical and five of them serious. Slaten said three others were treated at other hospitals and released.

The alleged shooter, Jared Lee Loughner, was wrestled to the ground by two bystanders at the scene and taken into police custody. Little was immediately known about his motive. Loughner, 22, apparently posted several videos on YouTube, which featured mostly rambling text that discussed the invention of a new currency and a distrust of the government.

Loughner allegedly targeted Giffords as well as her district director and shot indiscriminately at staffers and others standing in line to talk to the congresswoman, said Mark Kimble, a communications staffer for Giffords.

“He was not more than three or four feet from the congresswoman and the district director,” Kimble said, describing the scene as “just complete chaos, people screaming, crying.”

Pima County Sheriff’s office said another man, described as dark haired and between 40 and 50 years of age was being sought as a “person of interest” who caught on video by a security camera near the scene.

Hours after the shooting, police in Tucson had to deal with a suspicious package.

Police department spokesman Lt. Fabian Pacheco said an officer checking Giffords’ office had found “a real strange” device that he said resembled a coffee can and had writing on it. Pacheco would not disclose what the writing said. A bomb squad had been working to render the device safe before the loud noise was heard. A loud noise rattled more than 100 people attending a candlelight vigil on Saturday near the headquarters of Rep. Giffords.

Videos Of Alleged Shooter

Saturday’s shooting has prompted outrage and speculation about the possible motive of the man who allegedly opened fire in a Safeway grocery store parking lot on Saturday.

Pima County Sheriff Clarence Dupnik said there’s reason to believe Loughner has “a mental issue” and described him as “unhinged.”

“As we understand it, there have been law enforcement contacts with the individual where he made threats to kill,” Dupnik said during a news conference Saturday evening. But he wouldn’t say who those threats were aimed at.

Giffords herself warned months ago that the verbal assaults were beyond the pale and could have dire results.

Loughner apparently was the source of six videos posted on YouTube in recent months. The most recent one, titled “America: Your Last Memory In A Terrorist Country!” shows a hunched-over man in a hooded sweatshirt burning an American flag in a desert landscape. The soundtrack is a song called “Let the Bodies Hit the Floor.”

Other videos have long, written tirades against government, currency and grammar.

In one, he says, “The majority of citizens in the United States of America have never read the United States of America’s Constitution.”

In another, he says “I can’t trust the current government because of the ratifications: The government is implying mind control and brainwash on the people by controlling grammar.”

At one point, he called Arizona the capital of anger, hatred and bigotry, and he said unbalanced people can respond to that in dangerous ways.

In one of the videos, Loughner describes himself as a U.S. military recruit, but an Army spokesman said he attempted to enlist and was rejected.

Loughner “was a guy in high school who definitely had his opinions on stuff and didn’t seem to care what people thought of him,” high school classmate Grant Wiens, 22, told The Associated Press.

Wiens also said Loughner used to speak critically about religion. He also talked about how he liked to smoke pot.

“He wasn’t really too keen on religion it seemed like,” Wiens said. “I don’t know if floating through life is the right term or whatever, but he was really just into doing his own thing.”

The MySpace page, which was removed within minutes of the gunman being identified by officials, included a mysterious “Goodbye friends” message published hours before the shooting and exhorted his friends to “Please don’t be mad at me.”

Lynda Sorenson said she took a math class with Loughner last summer at Pima Community College’s Northwest campus and told the Arizona Daily Star he was “obviously very disturbed.”

“He disrupted class frequently with nonsensical outbursts,” she said.

In October 2007, Loughner was cited in Pima County for possession of drug paraphernalia, which was dismissed after he completed a diversion program, according to online records.

Second Man Sought

Dupnik said Saturday that law enforcement has reason to believe Loughner came to the grocery store with another individual who may in some way be involved, but was not himself a shooter.

Later Dupnik gave a bit more information, saying that the other suspect is a white male, possibly in his 50s. A photograph of the man was later released, but police said they don’t know his identity.

The sheriff’s office said early Sunday that authorities were still seeking for a second man “possibly associated with the suspect” who was filmed by a video camera near the scene of the shooting and is wanted for questioning.

The second man was last seen wearing blue jeans and a dark blue jacket, the sheriff’s department said in a statement.

Politicians React

In Washington, lawmakers from both parties were deeply shaken. The House’s newly installed Republican leaders postponed Wednesday’s scheduled vote to repeal the new health care law. That divisive issue was at the center of the harshest criticisms of Giffords and many other Democrats for the past two years.

House Speaker John Boehner, speaking briefly on Sunday morning, asked for lawmakers and their staff to keep Giffords in their prayers.

He said the shooting of the Arizona Democrat was an “inhuman act” that “should not and will not deter us from our calling to represent our constituents and fulfill our oaths of office.”

“An attack on one who serves, is an attack on all who serve,” he said. “Such attacks have no place in our society.”

State Democratic Party Chairman Don Bivens said words could not express his shock and devastation over the attack.

“If we can’t keep our public servants safe we are a serious peril,” he said.

Democratic Rep. Raul Grijalva, whose Arizona district shares Tucson with Giffords’ district. But he said the nation must assess the fallout of “an atmosphere where the political discourse is about hate, anger and bitterness.”

NPR’s Brian Naylor, Ari Shapiro and Audie Cornish and KUAZ’s Buzz Conover contributed to this report, which contains material from The Associated Press. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.