(CNN) -- Six people were killed and 12 others wounded -- including U.S. Rep. Gabrielle Giffords -- in a shooting outside a Tucson, Arizona, grocery store where the congresswoman was holding a constituent meeting, police said.
The suspect in the Saturday morning shooting was in custody, according to U.S. Capitol Police, who did not identify the alleged gunman. A law enforcement source said he was a white male who is 22 or 23 years old.
Police have also recovered a handgun, according to senior law enforcement sources.
Dr. Peter Rhee of University Medical Center in Tucson, said Giffords, 40, had undergone surgery for a single gunshot to the head that passed through her brain. Giffords was among five patients listed in critical condition, Rhee said.
A 9-year-old girl was among the dead, Rhee said.The child, whose identity had not been released, was pronounced dead at the hospital.
A federal judge, identified as Chief Judge John Roll by Homeland Security Secretary Janet Napolitano, also was injured in the attack.
At least two victims with gunshot wounds were transported at Northwest Medical Center, according to spokesman Richard Parker.
Rick Kastigar, bureau chief for the Pima County Sheriff's Department, declined to comment on a possible motive for the attack.
Tucson police Chief Roberto Villasenor said his department had secured the homes and offices of some unnamed federal officials as a "precautionary measure".
The U.S. Capitol Police said it has advised other members of Congress to take "reasonable and prudent precautions regarding their personal safety and security," the agency said in a statement.
An FBI spokesman said the agency was sending agents to the scene.
Rhee said that Giffords had been moved to intensive care from surgery and said he is "very optimistic" about her recovery.
"I'm about as optimistic as it can get in this situation," he said.
He said the next 24 hours will be key for determining the extent of her recovery.
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.
Roll is a 1991 Bush appointee to the federal bench. The chief judge on the U.S. District Court for Arizona, he is originally from Pennsylvania.
Giffords' colleagues in Congress reacted quickly to the shooting.
Sen. John McCain, R-Arizona, said the attack was committed by a "wicked person who has no sense of justice or compassion".
"Whoever did this, whatever their reason, they are a disgrace to Arizona, this country and the human race, and they deserve and will receive the contempt of all decent people and the strongest punishment of the law," he said.
President Barack Obama called the attack an "unspeakable tragedy".
"We do not yet have all the answers," he said. What we do know is that such a senseless and terrible act of violence has no place in a free society".
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".
Rep. Chellie Pingree, D-Maine, said an incident like this could have a chilling effect on the frequent weekend listening post sessions many members of Congress have with their constituents.
"It does give you pause," Pingree said. CNN
