Crews worked Monday to neutralize and remove chlorine gas (search) still leaking from a railcar damaged in a train wreck that killed nine people last week and injured hundreds more.

A day after patching a breach in the car, crews mixed the toxic gas with sodium hydroxide (search) to turn it into liquid bleach, making it safer to pump it out of the railcar, said Thom Berry, state Department of Health and Environmental Control spokesman.

The gas killed nine people, including six workers at a nearby textile plant, and sickened more than 250 after the Norfolk Southern (search) train slammed into parked railcars early Thursday.

Thousands of nearby residents were expected to remain evacuated until Wednesday at the earliest. Two other cars containing chlorine gas remain at the crash site, but are not leaking, officials said. They will be emptied before being moved, Berry said.

Of the more than 250 people who received treatment after being exposed to the gas, 33 remained hospitalized Monday, including at least two in critical condition, officials said. One of those still hospitalized was the train's conductor; the engineer died at a hospital.

An overnight curfew remained in effect for people who refused to leave their homes in the affected area.

Investigators determined that the three-man crew that parked a two-car train on a side rail failed to switch the tracks back to the main rail. That sent the oncoming train hurtling into the parked train.

Graniteville, about 10 miles from the Georgia state line, is in an area where rail switches are controlled manually, said Debbie Hersman, a National Transportation Safety Board (search) spokeswoman.

The investigation showed the crew on the moving train applied its brakes before hitting the parked train, she said.

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