Doctors treating wounded Rep. Gabrielle Giffords provided an optimistic update Sunday about her chances for survival, saying they are "very, very encouraged" by her ability to respond to simple commands along with their success in controlling her bleeding.

The FBI said law enforcement could bring charges against the gunman as early as Sunday, and House Speaker John Boehner said normal House business this week has been postponed in the aftermath of the shooting.

Surgeons said a bullet went through Giffords' head on the left side of the brain, but she is still able to respond nonverbally to commands such as squeezing a hand or showing two fingers. They credited several reasons for her survival, including good luck and the fact that paramedics got her to surgeons quickly — in under 40 minutes.

"This is about as good as it is going to get," said Dr. Peter Rhee, a trauma surgeon. "When you get shot in the head and the bullet goes through your brain, the chances of you living is very small and the chances of you waking up and actually following commands is even much smaller than that. Hopefully it will stay that way."

Surgeons worked to reduce pressure from swelling in her head by removing bone fragments, and they also removed a small amount of badly damaged brain. Giffords cannot speak because she is on a ventilator.