"We're all interested in seeing that, in light of this tragic incident, that things still are able to move forward and that Pakistan is able to continue down a democratic path," Casey said.
Condoleezza Rice, the secretary of state, on Friday called Bhutto a "champion for democracy" and urged that reform continue.
"This is a day of great tragedy, great mourning. She was a champion for democracy. She was a courageous woman," she said after signing a book of condolence for Bhutto at Pakistan's embassy in Washington.
The distant tragedy has not gone ignored by US presidential hopefuls, who have been swift to condemn Bhutto's assassination.
Many have used the incident to lay out their own foreign policy regarding Pakistan and to make pointed attacks on their rivals.
Hillary Clinton, the Democratic presidential front-runner, called on Friday for an independent, international probe into Bhutto's murder.
She also said that the Bush administration's policy of giving General Pervez Musharraf, the Pakistani president, "a blank cheque" for aid had failed.
Barack Obama, the Illinois senator currently locked in a close fight with Clinton for the Democratic nomination, agreed with Clinton's comment on funding, remarking that the US had "poured billions of dollars in support to President Musharraf - and he has not focused on dealing with the terrorist threat that is growing".
However, both later traded accusations that the other was using the assassination for political gain.
Wesley Clark, a former Nato commander and Clinton supporter, criticised Obama's campaign after the senator's chief strategist implied Clinton's support for the war in Iraq had boosted al-Qaeda and called for "leadership, not politics".
'A terrible day'
While campaigning in Iowa, John Edwards, a former North Carolina senator, said he had spoken to Musharraf since Bhutto's death. He called for an internationally led investigation into the killing.
"I urged him to continue the democratisation process because of how important it is to the Pakistani people and how important it is to his country," Edwards said.
The failure to protect Ms Bhutto raises a lot of hard questions for the government and security services that must be answered
Bill Richardson, a Democratic candidate and former UN ambassador, called for a halt to US aid to Pakistan until Musharraf left office and full democracy was restored.
"Not one penny more ... until Musharraf is gone and the rule of law is restored," he said.
And Joseph Biden, chairman of the US senate foreign relations committee, said he had recently written to Musharraf, asking for greater protection for Bhutto.
He said: "This is a terrible day. I am convinced Ms Bhutto would have won free and fair elections next week.
"The failure to protect Ms Bhutto raises a lot of hard questions for the government and security services that must be answered."
For his part, Dennis Kucinich, another Democratic hopeful, issued a statement describing Bhutto as "dedicated to the people of Pakistan".
He called on the US to change its policy towards Pakistan and "stop adding fuel to the fire".
Impact on Republics
Like the Democrats, several Republican candidates took the opportunity, while commenting on Bhutto's death, to bolster their foreign-policy credentials.
Senator John McCain said that elections in Pakistan should go ahead, but acknowledged it would be hard for Pakistan's opposition to rally around an alternative candidate.
|Republicans Huckabee [with McCain] cautioned candidates against ''political games'' [EPA]|
"My theme has been throughout this campaign that I'm the one with the experience, the knowledge, the judgement," he said.
"So perhaps it may serve to enhance those credentials to make people understand that I've been to Pakistan, I know Musharraf, I can pick up the phone and call him. I knew Benazir Bhutto."
Mitt Romney, former governor of Massachusetts, expressed concerns about whether Musharraf would be able to control the unrest currently sweeping the country.
"I'm not concerned about the quality of his character, but I am concerned about the quality of his judgment in a setting like this," he said.
Mindful of the Democratic candidates' barbed remarks, Mike Huckabee, the former governor of Arkansas, warned his rivals not to use Bhutto's death to play "political games".
But he had earlier been left embarrassed when, in reaction to Bhutto's death, he asked whether the state of emergency in Pakistan should be lifted, when in fact this had already happened.
Meanwhile, Rudolph Giuliani, mayor of New York City during the 11 September attacks in 2001, cast his reaction as a reminder of how the threat of terrorism was "an enemy of freedom".
"We must redouble our efforts to win the terrorists' war on us," he said.