A week after chlorine gas (search) leaked from a train wreck and killed nine people in this mill town, officials will let about half the 5,400 residents displaced by the accident return home Thursday morning.

Air quality tests show chlorine has returned to normal levels on the edges of the evacuation area, which extended a mile from the crash site, according to the Aiken County Sheriff's Office (search).

Conditions remain too dangerous to allow people back into homes and businesses closer to the crash site, near the Georgia line and about 60 miles southwest of Columbia.

"Common sense and briefings tell us at ground zero it will take folks longer to get back home, but not because of contamination — it's because of the process," Sheriff Michael Hunt said.

Nearby residents have not been allowed to return home since hours after a Norfolk Southern train slammed into a parked train on a side track early Jan. 6. The crash punched a fist-sized hole in a tanker carrying chlorine gas.

The deadliest train wreck involving hazardous material in nearly three decades, it also injured more than 250 people. Six of the dead worked at a nearby textile mill; some of their co-workers survived by climbing up to the roof.

Any residents worried about returning home can arrange to have environmental officials inspect their homes, deputies said.

Workers unloaded the last of the chlorine Wednesday from two undamaged tankers and placed a steel patch over the hole in the damaged car. Next, crews will clean the tankers so they can be removed from the crash site, officials said.

Gov. Mark Sanford (search) asked President Bush and the U.S. Small Business Administration to make Aiken County eligible for federal financial assistance and low-interest loans.

State environmental officials, meanwhile, were assessing reported fish kills in a nearby pond.

The railroad company was continuing to operate an assistance center in Aiken to help residents who were displaced. The company said Wednesday that more than 2,500 people had received payments.