A third group has claimed responsibility for the deadly bombing of U.N. headquarters in Baghdad, posting the claim on a Web site known as a clearinghouse for Islamic political thought.

The statement — also quoted at length Monday in the London-based Arab newspaper Al-Hayat — is signed "Abu Hafs el-Masri Brigades." The shadowy group takes its name from the alias of Mohammed Atef (search), Usama bin Laden's top deputy who was killed in a U.S. airstrike in Afghanistan in November 2001.

U.S. officials in Washington said they could not authenticate the claim, however, and it remained unclear if the group exists or has any link to the Al Qaeda (search) terror network.

The claim was first posted Sunday on the site of London-based Saudi dissident Saad Al-Fagih's Movement for Islamic Reform in Arabia.

Al-Fagih deleted the posting Monday, but it reappeared on the site page, which is similar to a chat room. He said he would continue to remove it, and that he regularly deletes postings deemed offensive or which lacked authenticity.

He said the language was clearly that of Muslim extremists but it was impossible to discover where it originated.

"We carried out two operations, one in Afghanistan and the other in Iraq," said the statement, dated Aug. 19, the day of the U.N. attack.

It described an assassination attempt last year on Afghan President Hamid Karzai (search) and the U.N. headquarters bombing, which killed at least 23 people including top U.N. envoy Sergio Vieira de Mello (search).

With little evidence that Abu Hafs el-Masri Brigades exists as a group, Islamic terrorism expert Rohan Gunaratna told The Associated Press he believed the statement was a hoax.

Gunaratna, the author of "Inside Al Qaeda," said he thinks the statement was posted by "Islamists sympathetic to Al Qaeda" but not anyone who actually carried out the attack.

At least two other unconfirmed claims of responsibility have surfaced.

The first was a statement sent to the Arab television station Al-Arabiya last week, signed by a previously unknown group, the "Armed Vanguards of the Second Muhammad Army." The other was in a tape broadcast Saturday by the Lebanese television station LBC from a group calling itself "Muhammad's Army."