It's become a strongly held belief by some in the storm zone — the idea that the destruction of New Orleans’ heavily poor, heavily black Ninth Ward was neither an accident nor an act of nature.

Dyan French, also known as “Mama D,” is a New Orleans Citizen and Community Leader. She testified before the House Select Committee on Hurricane Katrina on Tuesday.

“I was on my front porch. I have witnesses that they bombed the walls of the levee, boom, boom!” Mama D said, holding her head. “Mister, I'll never forget it.”

“Certainly appears to me to be an act of genocide and of ethnic cleansing,” Leah Hodges, another New Orleans citizen, told the committee.

Similar statements, sometimes couched as rumors, have also been voiced by Louis Farrakhan, leader of the nation of Islam, and director Spike Lee.

“I don't find it too far-fetched,” Lee said in a recent television interview, “that they try to displace all the black people out of New Orleans.”

Harvard's Alvin Pouissant says such conspiracy theories are fueled by years of government neglect and discrimination against blacks: slavery, segregation and the Tuskegee experiments, during which poor blacks were used to test the effects of syphilis.

“If you're angry and you've been discriminated against,” Pouissant says, “then your mind is open to many ideas about persecution, abandonment, feelings of rejection.”

Video: Heated exchange The latest theory is partly rooted in historical fact. In 1927, the levees were bombed to save parts of the city, and black neighborhoods were inundated.

But independent engineers investigating levee failures during Katrina say that's not what happened this time.

Prof. Robert Bea, from the University of California, Berkeley, studied the levee failures and his team issued a preliminary report.

“We didn’t find any evidence that would indicate explosions,” says Bea.

New Orleans columnist Lolis Eric Elie says the federal government badly neglected black Americans during Katrina, but he does not believe the levees were blown up.

“One of the problems with that theory,” Elie says, “is that there were a whole lot of other areas of the city, including some that are predominantly white, where there was flooding.”

Harvard's Pouissant also says that latching on to conspiracy theories is a way for the powerless to cope with terrible losses — incendiary claims born of an enormous tragedy.

Lisa Myers is NBC’s Senior Investigative Correspondent