Some U.S. intelligence experts and analysts said that there are so many tangled alliances between the extremist groups and Pakistani government agencies that it would be virtually impossible to get to the bottom of who killed Bhutto unless the perpetrators came forward -- with proof. The FBI has offered to send investigators, but Pakistan has not responded, FBI spokesman Richard Kolko said.
Markey also wondered whether U.S. officials should trust Pakistan to aggressively investigate the slaying. "I have zero confidence that the Pakistan government will get to the bottom of this, if they want to or if they don't want to, no matter who is actually responsible for it," he said.
The extremist groups, Markey said, have "their tentacles already extended into the organs of the Pakistani state, which is what makes this so troubling."
Stanzel told reporters in Crawford, Texas, that it was "up to the Pakistani officials" to determine who killed Bhutto. He declined to say whether the Bush administration believed Pakistan was up to the task.
Bhutto had suggested that alliances between extremists and the government had put her country in a stranglehold, and that some combination of those forces might someday kill her.
"I have long claimed that the rise of extremism and militancy in Pakistan could not happen without support from elements within the current administration," Bhutto wrote in a commentary last month for CNN.
Before her return to Pakistan, Bhutto said she feared that retired army officers were plotting to assassinate her. In an interview with Britain's Guardian newspaper, she noted that Mahsud, the Taliban commander, had threatened to send suicide bombers against her. But she said real danger came from extremist elements within the country's military establishment that were opposed to her return.
"I'm not worried about Baitullah Mahsud, I'm worried about the threat within the government," she told the Guardian. "People like Baitullah Mahsud are just pawns. It is those forces behind him that have presided over the rise of extremism and militancy in my country."
Pakistani officials angrily denied such allegations. They did so again after Bhutto narrowly escaped injury Oct. 19, when suicide bombers attacked her homecoming parade, killing more than 140 people. No group has claimed responsibility for that attack.
But Bhutto described it as an attempt to silence her and her opposition candidacy, and called for international assistance in identifying the perpetrators. The Musharraf government declined to seek outside help, and the investigation appears to have made little progress.
On Thursday, Pakistani officials noted that radical extremists had also displayed an interest in going after Musharraf and his loyalists. The groups have launched several failed assassination attempts against Musharraf. And in recent weeks, suicide bombers have repeatedly targeted military and intelligence targets in Pakistan, including the military garrison in Rawalpindi where Musharraf stays.
Times staff writer James Gerstenzang in Crawford, Texas, contributed to this report.