WASHINGTON (CNN) -- The FBI and the Department of Homeland Security issued a bulletin Thursday citing an alleged claim of responsibility by al Qaeda for former Pakistani Prime Minister Benazir Bhutto's assassination, a DHS official told CNN.
An Italian news agency says al Qaeda No. 2 Ayman al-Zawahiri began planning Bhutto's killing in October.
But such a claim has not appeared on radical Islamist Web sites that regularly post such messages from al Qaeda and other militant groups.
The source of the claim was apparently Italian news agency, Adnkronos International (AKI), which said that al Qaeda Afghanistan commander and spokesman Mustafa Abu Al-Yazid had telephoned the agency to make the claim.
"We terminated the most precious American asset which vowed to defeat [the] mujahadeen," AKI quoted Al-Yazid as saying.
According to AKI, al Qaeda No. 2 Ayman al-Zawahiri set the wheels in motion for the assassination in October.
One Islamist Web site repeated the claim, but that Web site is not considered a reliable source for Islamist messages by experts in the field.
The DHS official said the claim was "an unconfirmed open source claim of responsibility" and the bulletin was sent out at about 6 p.m. to state and local law enforcement agencies.
The official characterized the bulletin as "information sharing."
Ross Feinstein, spokesman for Director of National Intelligence Mike McConnell, said the U.S. intelligence community is monitoring the situation and trying to figure out who is responsible for the assassination.
"We are not in a position to confirm who may be responsible," Feinstein said.
Feinstein said that the intelligence community "obviously analyze(s) open source intelligence," but he would not say whether the community believes the claim has any validity.
For now, he said, there is "no conclusion" as to who may be responsible.
Earlier, DHS spokesman Russ Knocke said Bhutto's assassination had not prompted "any adjustments to our security posture."
"Of course, we continue to closely monitor events as they unfold overseas," he said. E-mail to a friend
CNN's Jeanne Meserve contributed to this report.