The National Transportation Safety Board is nearing the end of the fact-finding phase of its investigation into the Howard Street train tunnel derailment and fire, but it has not yet determined what caused the five-day inferno that hobbled the city.

NTSB officials said yesterday they have completed several key steps of the investigation into the accident July 18 last year and should release the public docket, or statement of facts, early next year. A final report, which would include the probable cause, would come months after that.

"We're not at a point where we can make any conclusions," said NTSB spokesman Keith Holloway. While accident investigations generally take a year, he said this one has proven more complicated. "The issues involved - the fact that this included a pipeline and a tunnel and a fire - it's a little more detailed."

So far, the NTSB has completed a computer simulation of the derailment using information gathered from the train's event recorder, such as the speed of the train and when the brakes were applied.

The NTSB has also completed the metallurgical evaluations of rail within the tunnel and of the cast iron water pipe above the tunnel, which fractured and flooded the Howard Street corridor. It's still unclear which came first - the train derailment or the water pipe rupture.

Investigators have studied stress points and breaks along the rail and the pipe to help determine that.

The NTSB also has enlisted the help of the Army Corps of Engineers to study the tunnel substructure and the Nuclear Regulatory Commission to evaluate the effects of a fire in a tunnel. The National Institute of Standards and Technology and the Southwest Research Institute are also contributing.

The CSX train entered the west end of the tunnel near Camden Yards. About halfway through the tunnel, the train broke apart and 11 cars derailed. One of the derailed cars - a tanker carrying tripropylene - was punctured. The chemical caught fire and set ablaze seven cars carrying paper products.