Barely a third of the houses ruined by Hurricane Katrina have been torn down, leaving at least 9,000 still rotting while the pace of bulldozing has slowed, city and federal records show.

The houses -- empty and some untouched since Katrina came ashore 18 months ago -- are a lingering symbol of the storm's devastation and one of the biggest obstacles to the city's rebirth. So many still must be razed before Katrina's mess is fully cleared that the U.S. Army Corps of Engineers estimates the debris could fill the Empire State Building several times.

The work continues to slow down. The corps knocked down 118 houses in February, compared with 612 in December, an agency report said.

"Naturally I'd like to see us farther along, but I think we're months behind, not years behind," said Michael Park, director of the corps' recovery field office.

After Katrina struck in August 2005, federal and city governments estimated 15,000 homes in New Orleans needed to be razed.

Park said he expects demolition to be finished by the end of the year.