BOGALUSA, La. (AP) _ Workers began neutralizing the toxic chemical in a ruptured railroad tank car today, but the deadly fumes dimmed hopes for a quick all-clear for 1,500 people forced from their homes.

Fumes of nitrogen tetroxide prompted emergency crews to evacuate the command post on the city's baseball field. People in towns south of Bogalusa were advised that wind from the north might blow the fumes their way.

Roads into Bogalusa, about 60 miles north of New Orleans, were closed. So were the city's nine public schools and a Catholic school.

Three National Transportation Safety Board investigators headed to Bogalusa to find out what caused the accident, spokesman Pat Cariseo said.

The railroad car at the Gaylord Chemical Corp. plant in the heart of Bogalusa, about 60 miles north of New Orleans, began leaking around 4 p.m. Monday and imploded with a bang about 45 minutes later, Washington Parish Sheriff's Deputy Chris Nicolais said.

The implosion released a pinkish-yellow cloud that hovered over the area. Crews sprayed water onto the gas, which helped dilute the chemical. The fumes prompted hundreds of people to go to hospitals Monday, complaining of burning throats.

That cloud had dissipated by early today, but fumes were still a problem this morning.

``The gas is really, really bad stuff. It's fatal if inhaled,'' state Trooper Russell Mayfield said. Doctors went on New Orleans TV stations to advise anyone exposed to seek medical attention.

More than half the town's 16,000 residents live in the evacuation area. While some fled to towns and hospitals as far as 20 miles away, others stayed home with their windows tightly closed.

``They all have the same complaints: shortness of breath, eyes burning, throats burning,'' said state Rep. Jerry Thomas, a doctor at Riverside Medical Center in Franklinton, 20 miles from Bogalusa. ``It looks like a temporary thing for all of them. Some are receiving oxygen; some are just taking breaths and it's going away.''

The cloud that formed Monday was around 200 yards long, said Benton Lively, who lives near the plant.

``It was a big pink cloud, bright pink, like somebody took a crayon and used it over the mill. The pink later gave way to orange as the cloud grew darker, then began to diminish,'' Lively said.

Nitrogen tetroxide is used in rocket fuel but it was used at Gaylord's paper mill _ adjacent to the chemical plant _ to make heavy brown paper for boxes and bags.

It was unclear how many people sought medical attention.

Police Capt. Ernie Wells said about 300 people were treated at Bogalusa Community Medical Center, including two admitted to intensive care, and another 100 at Washington-St. Tammany Regional Medical Center, nearly a mile from the mill.

It also was unclear when evacuees would be able to go home.

One 88-year-old woman did not evacuate willingly.

``The law brought me _ they arrested me because I didn't want to leave,'' a flustered Erma Haik said early today at a Baptist church just outside the evacuation zone. ``They put me in the car and I was hollering.''