On Thursday, the U.S. National Transportation Security Board (NTSB) released a preliminary report on the derailment of a train carrying hazardous chemicals in East Palestine, Ohio, earlier this month.

The NTSB said the Norfolk Southern train passed three hot bearing detectors (HBD) on its trip before the derailment on the night of Feb. 3. The function of the HBD is to detect overheated bearings and provide audible real-time warnings to train crews.

One of the HBDs transmitted a critical audible alarm message instructing the crew to slow and stop the train to inspect a hot axle. After hearing the warning, the train engineer increased the dynamic brake application to further slow and stop the train and the crew later observed fire and smoke and notified the dispatcher of a possible derailment.

The derailed equipment included 11 tank cars carrying hazardous materials that subsequently ignited, fueling fires that damaged an additional 12 non-derailed railcars.

First responders implemented a one-mile evacuation zone surrounding the derailment site that affected up to 2,000 residents. There were no reported fatalities or injuries.

Responders mitigated the fire on Feb. 5. But five derailed "specification tank cars carrying 115,580 gallons of vinyl chloride" continued to concern authorities because the temperature inside one tank car was still rising. This increase in temperature suggested that the vinyl chloride was undergoing a polymerization reaction, which could pose an explosion hazard.

Vinyl chloride is a colorless gas that burns easily. It does not occur naturally and must be produced industrially for its commercial uses.Vinyl chloride exposure is associated with an increased risk of a rare form of liver cancer as well as primary liver cancer, brain and lung cancers, lymphoma, and leukemia.

The NTSB said that responders scheduled a "controlled venting" of the five vinyl chloride tank cars to release and burn the vinyl chloride and dug ditches to contain released vinyl chloride liquid while it vaporized and burned. The controlled venting began on Feb. 6, which discharged toxic and potentially deadly fumes into the air.

While residents were allowed to return to their homes in East Palestine two days later, they remain concerned about the handling of the incident as well as the health impact of exposure to those chemicals.

The hazardous material tank cars have been decontaminated, the NTSB said. The agency's investigators returned to Ohio earlier this week to "examine each hazardous material tank car, document damage, and secure evidence for laboratory analysis."

The ongoing investigation will focus on the wheelset and bearing; tank car design and derailment damage; a review of the accident response, including the venting and burning of the vinyl chloride; and railcar design and maintenance procedures and practices.

On Thursday morning, U.S. Transportation Secretary Pete Buttigieg visited East Palestine amid criticism that the response from the federal government to the incident has been slow and inadequate.

"This morning I'm in East Palestine, Ohio, to see the site of the Norfolk Southern derailment, hear updates from investigators, and meet first responders," Buttigieg tweeted.

Buttigieg's trip to East Palestine, a village in northeastern Columbiana County, Ohio, came a day after former U.S. President Donald Trump visited the site of the train derailment.

East Palestine Mayor Trent Conaway told reporters during Trump's visit that this has been a "very difficult situation" as the village is struggling to deal with the fallout of the derailment.

On Wednesday, Norfolk Southern announced that it would excavate the soil and replace the tracks in the derailment area as part of an enhanced remediation plan.

This corporation said it had excavated over 4,800 cubic yards of soil. Additionally, 1.7 million gallons, or approximately 200 tanker loads, of liquid had been collected for disposal.