Bhutto was killed in a suicide attack as she left a political rally Thursday in Rawalpindi. CNN said it was unclear whether she was shot or hit by shrapnel.

From his ranch in Crawford, Texas, Bush issued a statement, saying those responsible must be brought to justice.

"The United States strongly condemns this cowardly act by murderous extremists who are trying to undermine Pakistan's democracy," Bush said. "Those who committed this crime must be brought to justice.

"Mrs. Bhutto served her nation twice as prime minister. She knew her return to Pakistan earlier this year put her life at risk, yet she refused to let assassins dictate the course of her country."

Bush said the best way to honor her memory is for Pakistan to continue the democratic process "for which she so bravely gave her life."