The last tweet from U.S. Rep. Gabrielle Giffords said, "My 1st Congress on Your Corner starts now. Please stop by to let me know what is on your mind or tweet me later."

It sounded so innocuous.

Minutes later, mayhem broke out and Giffords was critically wounded by a shot to the head. Six people were killed, including federal judge John Roll, who had recently worked with Giffords on court issues. A total of 13 people were injured.

Also among the dead:

• Christina-Taylor Green, 9, who had recently been elected to student council and attended Saturday's event to learn more about government.

• Gabe Zimmerman, 30, Giffords' director of community outreach.

• Three retirees: Phyllis Schneck, 79; Dorwan Stoddard, 76; and Dorothy Morris, 76, whose last name was originally reported by law enforcement as Murray.

The shooting drew stunned reaction across Tucson. Groups gathered to hold prayer circles, while an impromptu shrine with candles and flowers cropped up outside Giffords' midtown office.

It also drew immediate national reaction, with new Republican House Speaker John Boehner calling it "a sad day for our country" and the White House calling it an "unspeakable tragedy."

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

The bullet exited Giffords' head cleanly. Dr. Peter Rhee, a surgeon at University Medical Center, said he was optimistic about her survival, and noted she was following commands.

The shooting occurred at a northwest-side Safeway supermarket, at North Oracle and West Ina roads, where Giffords, a Democrat, was holding one of her regular "Congress on Your Corner" events, which allow her to speak directly with constituents.

The gunman has been identified as 22-year-old Jared Loughner and Sheriff Clarence Dupnik said law enforcement is not convinced he acted alone.

He had several nonsensical postings on social-media sites, complaining about illiteracy in Congressional District 8 as well as rants about currency. Loughner was registered as an independent, according to the Pima County Recorder's Office.

Dupnik said the suspect had a "troubled past" and was uninjured but uncooperative.

"I've never been so shocked in my life as the events that happened today," Dupnik said. "It's not only a very sad day for Tucson and for the family, the friends of all the victims of this horrendous, senseless unbelievable crime - but it's a sad day for America."

Giffords has held several such mini-town-hall like events since first taking office in January 2007, but this was her first event since her re-election to a third term in November.

Mark Kimble, a former Tucson Citizen executive who has worked for Giffords since 2009, said he was chatting with fellow staffers and Judge Roll when he saw "this guy come running in from the south."

About 15 to 20 people were lined up to the north of where Giffords stood with chief of staff Ron Barber, between U.S. and Arizona flags, Kimble said.

"Even before he got in, he fired some shots, stopped briefly in front of Gabby and Ron and was no more than four feet, probably less, when he fired a number of shots at the two of them and then he stayed there for a couple seconds firing around, he continued north through the area where people were lined up waiting.

"It was a totally chaotic scene. Gabby and Ron were lying slumped against the front glass of the Safeway."

Kimble said he dived to the ground when the shooting began.

"All of these people who were there were trying to help out people who had a problem ... and this happens. This wasn't a political event. There was no controversy involved.

"To say 'It's a horrible event' is just inadequate."

Two bystanders wrestled the assailant to the ground and disarmed him. His semi-automatic pistol still had ammunition in it. Dupnik said others would probably have been shot had he not been restrained, adding it's a possibility that he also might have tried to kill himself.

Other members of Giffords' staff were among the wounded.

District director Barber had surgery for his gunshot wounds, and Pam Simon, Giffords' community-outreach representative, was also shot and injured. Both were expected to survive, Giffords spokesman C.J. Karamargin said.

Karamargin said there was no security at the event, although the office typically notified local law enforcement as a courtesy.

Giffords worked to be accessible to her constituents, Karamargin said.

"She always prided herself on reaching out to the people who elected her and it would compound this tragedy if that were to change," he said.

She has done 20 such "Congress on Your Corner" events, he said.

Last March, Giffords' Tucson office was vandalized after the House vote overhauling the nation's health-care system. With the social media circuits ablaze with furor over heated political rhetoric, including Sarah Palin's website that put Congressional District 8 in gun cross hairs, Dupnik multiple times complained about the vitriol invading the political scene. "We have become the mecca of prejudice and bigotry," Dupnik said Saturday.

"The anger, the hatred, the bigotry that goes on in this country is getting to be outrageous," he said. "Unfortunately, Arizona, I think has become sort of the capital."

In an interview with MSNBC after her office was vandalized, Giffords said she was not fearful but said she'd noticed an increase in heated political rhetoric.

However, former Republican Rep. Jim Kolbe, who represented the area before Giffords, said it would be inappropriate this early on to suggest any political motivation was involved.

"We've had shootings in schools and how many other places? If you have somebody who is deranged like this, we just don't know what was going on in his head," Kolbe said, adding he was "shocked and horrified" when he heard about the violence.

Kolbe, who also counted judge Roll as a friend, said while he didn't agree with Giffords on many recent votes, he credited her with doing a great deal of outreach in the district. "I don't know of anybody who had any hostility against Gabby - even those who disagreed with her. She was very bubbly, very thoughtful, always sending notes. It's just a terrible tragedy."

Giffords, elected to Congress in 2006, was the third woman in Arizona history elected to Congress and the first female Jewish member of Congress from Arizona.

She is a member of the fiscally conservative Blue Dog Coalition in the U.S. House and was named by a political magazine as one of the most centrist members of Congress, although some of her votes in the last congressional cycle drew the ire of the right.

Giffords, counted among the rising stars of the Democratic party and mentioned often as a prospective candidate for U.S. Senate, had just started the congressional session. She tweeted welcomes to new Republican Rep. David Schweikert of Arizona and had just sponsored legislation, with Republican Rep. Ron Paul of Texas, to cut congressional salaries.

Giffords, 40, married Cmdr. Mark E. Kelly, 46, a NASA astronaut and Navy pilot from New Jersey, in December 2007. Kelly flew in to be by his wife's side Saturday afternoon.

Giffords prides herself on her accessibility to the public, even though some of the meetings grew heated, particularly those focusing on the health-care overhaul. She held several large public meetings in 2009 about the legislation that drew vocal and rowdy protesters.

She told the Star that speaking with the people she represents is one of the most rewarding, and sometimes difficult, parts of her job.

Authorities are investigating or interviewing dozens of victims and witnesses.

The FBI is involved in the investigation. Obama sent Robert Mueller, the bureau's director, to Arizona to head the investigation.

Contact reporter Rhonda Bodfield at or 573-4243. Reporters Tom Beal and Josh Brodesky contributed to this story.