U.S. Rep. Gabrielle Giffords of Arizona was shot in the head Saturday, an aide was killed, and an unknown number of others were wounded when an assailant opened fire outside a grocery store as the Democratic lawmaker met with constituents, officials said.

C.J. Karamargin, a spokesman for Giffords, said the congresswoman was in surgery as of 1 p.m. local time and that an unspecified number of Giffords' staff members were wonded in the shooting. Karamargin said he had no other information on the conditions of the wounded or on the circumstances of the shooting.

Congressional officials said an aide to Giffords was killed, and an unknown number of others were wounded, including staffers to the lawmaker.

President Barack Obama called the shooting "an unspeakable tragedy" and that such "a senseless and terrible act of violence has no place in a free society.

U.S. Capitol police say the shooter is in custody. One official said he carried out the attack with an automatic weapon. The officials who described the events did so on condition of anonymity, saying they were not permitted to comment publicly.

"I am horrified by the senseless attack on Congresswoman Gabrielle Giffords and members of her staff," newly elected House Speaker John Boehner said. "An attack on one who serves is an attack on all who serve. Acts and threats of violence against public officials have no place in our society. Our prayers are with Congresswoman Giffords, her staff, all who were injured, and their families. This is a sad day for our country."

Three hours after the shooting, the L-shaped shopping center in Tucson was blocked off by police and had fire trucks and other vehicles in its parking lot that blocked the view of the store's front door. No shell casings could be seen from the area 500 yards (460 meters) from the store where reporters and photographers were kept.

Giffords, 40, was re-elected to her third term last November. She was a member of the Arizona House and Senate before coming to Washington.

Giffords tweeted shortly before the shooting, saying: "My 1st Congress on Your Corner starts now. Please stop by to let me know what is on your mind or tweet me later."

Giffords is married to astronaut Mark E. Kelly, who has piloted space shuttles Endeavour and Discovery. The two met in China in 2003 while they were serving on a committee there, and were married in January 2007.

Giffords was first elected to Congress amid a wave of Democratic victories in the 2006 election, and she won a narrow victory against a Republican backed by the ultraconservative tea party movement in the 2010 election.

She has been mentioned as a possible Democratic nominee in 2012 for the U.S. Senate seat held by Republican Jon Kyl, who has not said whether he'll run again, or for the governor's office in 2014.

The shooting comes amid a highly charged political environment that has seen several dangerous threats against lawmakers but nothing that reached the point of actual violence.

A San Francisco man upset with House Speaker Nancy Pelosi's support of health care reform pleaded guilty to threatening the Democratic congresswoman and her family, calling her directly on March 25 and threatening to destroy her Northern California home if she voted for health care reform.

In July, a California man known for his anger over left-leaning politics engaged in a shootout with highway patrol officers after planning an attack on the American Civil Liberties Union and another nonprofit group. The man said he wanted to "start a revolution" by killing people at the ACLU and the Tides Foundation.

Giffords herself has drawn the ire of the right, especially for her support of the health care bill from politicians like former Arizona Gov. Sarah Palin.

Her Tucson office was vandalized a few hours after the House vote to approve the health care law in March, with someone either kicking or shooting out a glass door and window.

Despite the animosity she has generated from the right, Giffords describes herself as a former Republican and a Blue Dog Democrat. The Blue Dogs are moderate centrist Democrats.

"You know, actually as a former Republican, you know, I consider myself someone who is pretty in the middle, I'm a blue dog Democrat, and one that is interested in making sure that our country maintains our prosperity and frankly, our superiority over other countries and that's where we look at these threat, obviously our defense budget, our level of education," she said in an interview with Fox News this week.

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AP Special Correspondent David Espo contributed to this report from Washington.