NRA should be proud! Arizona congresswoman among 12 shot at Tucson grocery

HIGHLIGHTS
NEW: Hospital: Rep. Gabrielle Giffords undergoing surgery for head wound
NEW: A federal judge is among the 11 others injured, a law enforcement source said
The shooting happened just after 10 a.m. MST at a Safeway store
Giffords was holding a constituent meeting at the store

(CNN) -- U.S. Rep. Gabrielle Giffords was critically wounded Saturday in a shooting at a Tucson, Arizona, grocery store in which at least 11 other people were injured, officials said.

Darci Slaten, a spokeswoman for University Medical Center in Tucson, said Giffords was undergoing surgery Saturday afternoon for a gunshot wound to the head.

A federal judge from Arizona was among the 11 others shot in the incident, a law enforcement source told CNN.

The shooting happened shortly after 10 a.m. MST, Pima County Sheriff's Department spokesman Jason Ogan said.

Giffords, 40, was holding a constituent meeting at the Safeway grocery store when the shooting occurred, according to a schedule posted on her website.

An FBI spokesman said the agency was sending agents to the scene, and two senior Obama administration officials said the White House is monitoring the situation. They said they believed that a congressional staffer may have been killed.

Pictures from the scene showed a Giffords banner hanging from the storefront.

At least two victims with gunshot wounds were transported at Northwest Medical Center, according to spokesman Richard Parker.

The conditions of the 11 others shot were not immediately known.

The motive for the shooting is unclear, Ogan said.

"We're just trying to sort this out right now," he said.

An employee of a nearby business, Jason Pekau, told CNN that he did not see the shooting, but heard "15 to 20 gunshots."

Giffords, a Democrat, was first elected in 2006. She has served as chairwoman of the House Space and Aeronautics Subcommittee and also holds seats on the House Science and Technology and Armed Services committees.

She won her third term in a closely contested race against a Tea Party-sponsored candidate and was one of three Democratic legislators who reported vandalism at their offices following the March vote on health care reform.

She is married to Navy Capt. Mark Kelly, a NASA astronaut who is scheduled to lead a space shuttle mission to the International Space Station.

House Speaker John Boehner said he was "horrified" by the shooting.

"An attack on one who serves is an attack on all who serve," he said. "Acts and threats of violence against public officials have no place in our society."

CNN's Dana Bash and Nick Valencia contributed to this report.