Why did the floodwalls surrounding New Orleans fail to protect the city from Hurricane Katrina? It’s an important question, and for the past several weeks, the Army Corps of Engineers has been giving a pat answer seemingly intended to head off further inquiry. According to corps officials, Congress authorized a flood-protection system that could only protect the city from a Category 3 storm. Katrina was a Category 4.

Case closed, right? Well, not exactly.

As it turns out, the corps’ explanation doesn’t hold water. And thanks to some solid reporting from the Washington Post and the New York Times, the corps’ own stonewall (against culpability) now appears to be crumbling. This morning, both papers dished out convincing front-page stories that contradicted the corps’ attempt to pass the buck to Congress.

“Louisiana’s top hurricane experts have rejected the official explanations for the floodwall collapses that inundated much of New Orleans,” begins the story in the Post, “concluding that Hurricane Katrina’s storm surges were much smaller than authorities have suggested and that the city’s flood- protection system should have kept most of the city dry.”

In other words, it wasn’t simply the strength of Katrina that led to the disastrous flooding. Rather, it was the weakness of the floodwalls. So what went wrong?

Relying primarily on the work of scientists at Louisiana State University’s Hurricane Center and its Natural Systems Modeling Laboratory, reporters Michael Grunwald and Susan B. Glasser lay out an alternate explanation. According to the Post, the scientists at LSU believe that the storm surge didn’t push water over the floodwalls, as initially suggested by the corps. Rather, the floodwalls failed to hold back the water because of some yet-to-be-determined structural problems.

“On a tour Tuesday, researchers showed numerous indications that Katrina’s surge was not as tall as the lakefront’s protections,” wrote the Post. “They showed a ‘debris line’ that indicates the top height of Katrina’s waves was at least four feet below the crest of Lake Pontchartrain’s levees. They also pointed out how the breached floodwalls near the lake showed no signs of overtopping — no splattering of mud, no drip lines and no erosion at their bases. They contended that the pattern of destruction behind the breaches was consistent with a localized ‘pressure burst,’ rather than widespread overtopping.”

“The center’s researchers said it is too early to say whether the breaches were caused by poor design, faulty construction or some combination,” added the Post.

Like the Post’s article, the story in the Times today also starts by casting doubt on the corp’s story about the flooding. “Since the storm, corps officials have said that there is a simple explanation for the devastation: Hurricane Katrina was a Category 4 storm and Congress authorized a flood control system to handle only a Category 3 storm,” notes the Times. “But federal meteorologists say that New Orleans did not get the full brunt of the storm, because its strongest winds passed dozens of miles east of the city. While a formal analysis of the storm’s strength and surges will take months, the National Hurricane Center said the sustained winds over Lake Pontchartrain reached only 95 miles per hour, while Category 3 storms are defined by sustained winds of 111 to 130 m.p.h.”

“This raises a series of questions about how the walls that failed were designed and constructed,” added the Times, “as well as whether the soil in some spots was too weak to hold them. Investigations by federal engineers and outside experts are just now beginning.”

In the meantime, reporters Christopher Drew and Andrew C. Revkin began advancing one possible hypothesis for the breaches. “The 2000 edition of the Army Corps of Engineers manual ‘Design and Construction of Levees’ says that the height of flood walls built on levees is an important factor in their ability to withstand a flood,” they reported in the Times. “For that reason, the manual says walls like those used in New Orleans ‘rarely exceed’ seven feet. But on two of the three canals where breaks occurred — the 17th Street and London Avenue canals - the concrete sections rise 11 feet above the dirt berms.”

Felix Gillette writes about the media for The New York Observer.