GRANITEVILLE, S.C. (AP) – Crews put a temporary patch Sunday on a railroad car that had been leaking toxic chlorine gas since a train wreck last week, while investigators looked into why a switching mechanism had been set to lead the train into railcars parked on a side track.

Nine people were killed and more than 250 were sickened by chlorine gas released when the tank car was damaged in the wreck of a Norfolk Southern train early Thursday. Thousands of nearby residents were to remain evacuated until Wednesday at the earliest.

Thom Berry, spokesman for the state Department of Health and Environmental Control, said workers would now focus on transferring the gas to a safer container and removing all the damaged railcars. About 16,000 gallons of sodium hydroxide has been safely removed from another railcar at the crash site, he said.

National Transportation Safety Board investigators have interviewed the three-man crew that had parked the cars on the side track Wednesday evening. Investigators said a switching mechanism wasn’t turned back to direct oncoming trains down the primary rail.

“We know that the switch was lined and locked for the siding,” said NTSB spokeswoman Debbie Hersman. “We won’t conclude anything today and we won’t speculate about the cause of the accident until we have gathered all the information.”

Hersman said the FBI is fingerprinting the switching mechanism to determine who operated it. She said there was no sign of outside tampering with the mechanism.

Rail switches are controlled manually in Graniteville, about 10 miles from the Georgia state line.

The area lacks sensors to notify approaching trains of track changes or other possible dangers, Hersman said.

Part of the investigation will also focus on the recent work history of the crews involved in the crash.

“Regardless of what anybody’s hours of service are, we always expect them to have a safe work site and to follow any rules that the company might have,” Hersman said.

More than 5,400 residents evacuated from their homes within a one mile radius of the site were told they would not be able to return until Wednesday at the earliest, Sheriff Michael Hunt said Sunday. An overnight curfew remained in effect for people who refused to leave their homes in the affected area.

Hunt said police arrested two men Saturday night in violation of the curfew and charged them with failure to obey a police officer.

Area schools were to remain closed through Tuesday.

State and federal environmental officials have been continually testing air quality in the area, finding either low levels or nothing at all a couple of blocks away from the site. Levels at the crash site were higher, which made capping the leaking car difficult, Berry said.