With college basketball championships under way around the country, the FBI has warned stadium operators of a possible suicide bomb attack at sporting events.

In a directive issued today, obtained by ABC News, the FBI said a posting on an extremist message board "advocated suicide attacks against sporting events as a cost-effective means of killing thousands of Americans."

The FBI and the Department of Homeland Security said they cannot confirm the credibility of the threat or whether the message is affiliated with al Qaeda.

The FBI said the Internet posting said the suicide attacks would be justified because the United States refused a truce offered by Osama bin Laden in his last videotaped statement, Jan. 19, 2006.

Posting Suggests Using American Suicide Bombers

According to the FBI bulletin, the author of the posting recommended using "three to five blond or black American Muslim suicide bombers." The FBI said the author suggested that homemade explosives be hidden under their winter clothing.

The posting recommended, according to the FBI, that one suicide bomber detonate inside the stadium and the others detonate at exit gates as spectators were fleeing.

"The combined explosions would create a panic that would kill far more spectators than the bombing alone," the FBI quotes the message as saying.

The FBI said it is unaware of any specific or credible plans to attack any sporting event but notes that terrorist groups have mounted attacks at sporting events in the past.

The FBI recommends that sports leagues and stadium operators review and coordinate security practices and emergency response procedures "to address this potential threat."