An overheated component on a railcar carrying plastic pellets was the likely cause of a Norfolk Southern Corp. train derailment earlier this month, federal transportation officials said Thursday, an incident that created an environmental mess in a small Ohio town and that the railroad and state and federal officials are still trying to clean up.

Before the train carrying several cars of hazardous chemicals derailed on Feb. 3, the crew received a warning in the train cab telling them to immediately slow and stop the train to conduct an inspection, according to a preliminary report from the National Transportation Safety Board. It was during the deceleration that the railcar wheel bearing failed, NTSB Chair Jennifer Homendy said during a press briefing.

By the time the roughly 150-car train stopped in East Palestine, the crew saw fire and smoke and notified authorities of a possible derailment, the NTSB report said. Before the derailment, the train was traveling eastbound at 47 miles an hour, below the maximum authorized speed of 50 mph, toward Conway, Pa.

Thirty-eight cars derailed, and fire damaged an additional 12 cars, according to the NTSB. Five tanker cars carried 115,580 gallons of vinyl chloride, a flammable and toxic gas used to make PVC pipe. Other railcars carried hazardous materials such as isobutylene and butyl acrylate. Days after the derailment, the railroad did a controlled burn of vinyl chloride from the tanker cars to prevent an explosion.

The initial fire was the result of the hot axle and the plastic pellets in the railcar, said Ms. Homendy, adding that there was no evidence that the three-person train crew had done anything wrong. She said the NTSB, so far, hasn’t identified any operational issues with the defect detectors or railroad tracks.

The NTSB said it would hold an investigative field hearing this spring in connection with the accident. The agency said its early findings are subject to change pending additional information gathered.

"This is a community that has been devastated," Ms. Homendy said at a press briefing. "They deserve to know what happened, how to prevent it from happening again. They deserve to have the right solutions."

The NTSB briefing showed that federal investigators, Norfolk Southern and others are still trying to piece together the circumstances that led to the derailment. While no injuries were reported in connection with the incident, residents of the town of about 5,000 near the Ohio-Pennsylvania border are still grappling with the fallout.

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The subsequent cleanup and ongoing health concerns have frustrated residents and local officials. Air, water and soil sampling is ongoing in the town, and federal authorities expect it to continue in the weeks to come. Residents, Ohio and Pennsylvania state governors and members of Congress have questioned whether appropriate safety measures were taken after the derailment and have scrutinized how the rail industry is regulated.

The temperature threshold at which Norfolk Southern detectors notify train crews of problems with rail equipment is among the lowest in the industry, Norfolk Southern said Thursday. Out of an abundance of caution, it is now inspecting all of its nearly 1,000 heat detectors, on top of regular inspections every 30 days, the company added. Norfolk Southern chief executive Alan Shaw has said that the railroad would pay for environmental remediation and monitoring and would stay to help East Palestine recover and thrive for as long as it takes.

"We will also work with the owners of the railcars on the integrity and safety of the equipment we use," the railroad said.

GATX Corp., a Chicago-based company that manufactured the railcar whose bearing overheated, declined to comment. The owner of the railcar couldn’t immediately be learned. Most railcars in the country are no longer owned by the railroads and are owned by railcar lessors and shippers.

Transportation Secretary Pete Buttigieg visited the cleanup site in East Palestine for the first time Thursday. Officials from Ohio and Pennsylvania, as well as federal officials from agencies including the Environmental Protection Agency and the Department of Health and Human Services, have been in the town in the days following the derailment.

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Mr. Buttigieg toured the wreckage of burned tanker cars that have yet to be cleared from the site and met with the village’s mayor, Trent Conaway, as well as residents and staff from federal agencies participating in the cleanup.

He also reiterated calls for tougher regulations on the rail industry, including increasing fines for safety violations and speeding up a timeline for requiring stronger tanker cars. He praised residents for their resilience following the accident and the national attention that has followed.

"I felt strongly about this and could have expressed that sooner," Mr. Buttigieg said. "I think the most important thing is making sure that the residents have what they need."

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The Association of American Railroads, a railroad-industry trade group, said Thursday that railroads will use the NTSB’s initial report to shape "a thoughtful, fact-driven approach to prevent another similar accident before it can occur elsewhere."

The NTSB will be studying the wheelset and the bearing believed to be the source of derailment in its laboratories. There are various causes for roller bearings to overheat, including fatigue cracking, water damage and mechanical damage, Ms. Homendy said. She added that bearings have a finite life of around 100,000 miles to 300,000 miles.

"You cannot wait until they fail," she said.

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The NTSB will also investigate the accident response, including how responders and the public received information about the derailment. While the public doesn’t need to know detailed information like specific dates and times of hazardous material tank cars traveling on any route, "they need to know, absolutely deserve to know, whether they live or work near a hazmat route," said Ms. Homendy.

Before the derailment, the train passed three monitors along the tracks that are designed to detect overheating equipment. Norfolk Southern has a protocol for crews to follow if the detector equipment records a temperature that exceeds a certain threshold, the NTSB said.

Two defect detectors, located 30 miles and 20 miles away from the one that sounded the alarm, didn’t record temperatures above Norfolk Southern’s established alarm thresholds. But when the train reached the third monitor, a bearing on the 23rd railcar measured at 253 degrees Fahrenheit above the ambient temperature, well above critical levels, the NTSB said.

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"Had there been a detector earlier, the derailment may not have occurred," said Ms. Homendy, adding that more investigations need to be done. Railroads currently have discretion to determine how much space there is between detectors as well as temperature thresholds before alarms are sounded for the train crew. Ms. Homendy said the NTSB will also look into those issues.

Kris Maher and Eric Bazail-Eimil contributed to this article.