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Former Pakistani Prime Minister and opposition leader Benazir Bhutto was assassinated in a suicide attack after speaking at a campaign rally in Rawalpindi, Pakistan on December 27, 2007. Bhutto's attacker shot her in the neck and chest and then blew himself up next to her car. She was 54. Bhutto is pictured with U.S. President Bush at the White House in a June 6, 1989 file photo in Washington. (UPI Photo/Cliff Owen/FILES)

The official said the bulletin -- issued Thursday by his department and the FBI -- cited Italian news agency Adnkronos International's report that al-Qaida Afghanistan commander and spokesman Mustafa Abu Al-Yazid claims the former Pakistani prime minister's assassination was planned and executed by al-Qaida, CNN reported Friday.

"We terminated the most precious American asset which vowed to defeat (the) mujahedin," Al-Yazid told the news agency.

He said al-Qaida's second-in-command, Ayman al-Zawahiri, began plotting the official's assassination in October.

However, the claim hasn't been repeated on radical Islamist Web sites that often post claims of responsibility for attacks from al-Qaida and other groups.

The Homeland Security official said the claim was "an unconfirmed open source claim of responsibility." He said the bulletin was sent Thursday to state and local law enforcement agencies.

Ross Feinstein, spokesman for U.S. Director of National Intelligence Mike McConnell, said the U.S. intelligence community is working to discover who was behind the attack.

"We are not in a position to confirm who may be responsible," Feinstein said.