NEW ORLEANS – The U.S. Army Corps of Engineers raced to patch New Orleans’ fractured levee system Tuesday, and residents were forced to decide yet again whether to stay or go as rapidly strengthening Hurricane Rita threatened to flood the city anew.

Rita, which strengthened to a Category 2 storm as it barreled past the Florida Keys into the Gulf of Mexico, could strengthen to a 131-mph-plus Category 4 and hit Texas by the end of the week, forecasters said.

But a slight turn to the north was possible, and engineers warned that even a glancing blow to New Orleans and as little as 6 inches of rain could swamp the city’s levees.

“The protection is very tenuous at best,” said Dave Wurtzel, the Army Corps official responsible for repairing the 17th Street Canal levee, whose huge breach during Katrina caused the worst of the flooding that wrecked the city.

Separately, Louisiana’s top hurricane experts rejected the official explanations for the floodwall collapses that inundated much of New Orleans, concluding that 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.

Scientists and engineers at Louisiana State University’s Hurricane Center have concluded that Katrina’s surges did not come close to overtopping those barriers. That would make faulty design, inadequate construction or some combination of the two the likely cause of the breaching of the floodwalls along the 17th Street and London Avenue canals .

“We are absolutely convinced that those floodwalls were never overtopped,” Ivor van Heerden, the hurricane center’s deputy director, said Tuesday.

In anticipation of another hurricane, the corps on Tuesday drove a massive metal barrier across the 17th Street Canal bed to prevent a storm surge from Lake Pontchartrain from swamping New Orleans again.

In addition, the corps had 800 giant sandbags weighing 6,000 to 15,000 pounds on hand just in case, and ordered 2,500 more to shore up low spots and plug any new breaches. It also was putting pumps and other materials where they might be needed.

Meanwhile, Mayor Ray Nagin and Louisiana Gov. Kathleen Blanco urged people along the Louisiana coast to be prepared to get out of Rita’s way. The federal government’s top official in the city, Coast Guard Vice Adm. Thad Allen, said the preparations in and around New Orleans included 500 buses for evacuation and enough water and military meals for 500,000 people.

“First it was come back, then it was go,” said Karen Torre, who returned to her Uptown home Tuesday to haul away debris and clean rotted food from her refrigerator before leaving again. “We’re just trying to do what they tell us and get a few things done in between.”

Even residents who have already been evacuated faced the prospect of being uprooted again. At the Cajun Dome in Lafayette, emergency officials arranged to take the 1,000 refugees from the New Orleans area out on buses if Rita tracks north.

“Nobody here even wants to hear the word ‘hurricane’ right now,” said Carlette Ragas, who has not been back to her home in Plaquemines Parish, south of New Orleans, since Katrina.

President Bush made his fifth trip to the Hurricane Katrina zone on Tuesday to meet with business and political leaders in Gulfport, Miss., and received a briefing in New Orleans on preparations for Hurricane Rita.

On Tuesday, less than 20 percent of New Orleans was under water, down from 80 percent after Katrina hit Aug. 29.

Using FEMA data, the National Low Income Housing Coalition on Tuesday estimated that Katrina destroyed or heavily damaged 302,000 homes. About 142,000 homes were lost in New Orleans alone.