CAIRO, Egypt — A message attributing a U.S. diplomat's slaying in Sudan to a previously unknown militant group has been posted in the chat room of a Web site used by militants, a group that monitors such sites said Friday.

The SITE Intelligence Group said it could not authenticate the claim, purportedly from a group calling itself Ansar al-Tawhid, or Companions of Monotheism.

The statement was posted in the discussion forum of a Web site commonly used by militants, not as an official statement on the site.

"Because there is a claim of responsibility, we chose to send it out to our subscribers," SITE Institute director Rita Katz told The Associated Press. Katz said that she had never heard of the group.

John Granville, an official for the U.S. Agency for International Development, was being driven home around 4 a.m. Tuesday when his car was cut off by another vehicle and came under fire, according to the Sudanese Interior Ministry. He was hit by five bullets and died after surgery. His driver, Abdel-Rahman Abbas, was also killed.

Sudanese officials insist the shooting was not a terrorist attack, but the U.S. Embassy said it was too soon to determine the motive.

FBI investigators are in Khartoum working with Sudanese security agencies to investigate the attack, which was the first assassination of a U.S. diplomat in Sudan since 1973.

In Washington, the State Department said it was not aware of any claim of responsibility in the slaying.

"At this point we don't have any clearer picture (of what happened)," spokesman Sean McCormack told reporters.