On Feb. 3, a train derailed in the village of East Palestine, setting off evacuation orders, a toxic chemical scare and a federal investigation.

This month a train carrying toxic chemicals derailed in eastern Ohio, igniting a fire that covered the town of East Palestine in smoke. Fearful of a major explosion, the authorities carved out an evacuation zone and carried out a controlled release of toxic fumes to neutralize burning cargo inside some of the train cars.

Residents feared for their health as concerns have mounted about the effect the derailment and the fire could have on the environment and other things including the transportation network.

Around 9 p.m. on Feb. 3, a train derailed in East Palestine, Ohio, a village of about 4,700 residents about 50 miles northwest of Pittsburgh. There were 150 cars on the route from Madison, Ill., to Conway, Pa. The National Transportation Safety Board, which is investigating, said on Feb. 14 that 38 cars derailed and a fire ensued, which damaged another 12 cars.

The train, operated by Norfolk Southern, had been carrying chemicals and combustible materials, with vinyl chloride, a toxic flammable gas, being of most concern to investigators. A huge fire erupted from the derailment, sending thick billowing smoke into the sky and over the town. Residents on both sides of the Ohio-Pennsylvania border were ordered to evacuate, as Gov. Mike DeWine of Ohio raised alarms about a possible explosion.

Local and federal officials started an investigation that involved the N.T.S.B. and the Environmental Protection Agency. The N.T.S.B. said its investigation included examining tank car fittings, the locomotive event data recorder and surveillance video from a residence that showed what appeared to be the failure of a wheel bearing moments before the derailment.

Has there been harm to the environment?

The derailment has caused concerns about air, soil and water pollution.

On Feb. 10, the E.P.A. said that about 20 rail cars were reported to have been carrying hazardous materials. Chemicals including vinyl chloride, butyl acrylate, ethylhexyl acrylate and ethylene glycol monobutyl ethers were “known to have been and continue to be” released to the air, surface soil and surface waters, the E.P.A. said.

On Feb. 12, the E.P.A., after monitoring the air, said it had not detected contaminants at “levels of concern” in and around East Palestine, although residents might still smell odors. Working with Norfolk Southern and the Columbiana County Emergency Management Agency, the E.P.A. had screened the air inside about 290 homes as of Feb. 13, and said it had not detected vinyl chloride or hydrogen chloride, which could cause life-threatening respiratory issues.

Precautionary measures were being taken in the wider region, particularly in states using water from the Ohio River.

The West Virginia subsidiary of American Water, which provides water services in 24 states, said on Feb. 12 that it had not detected any changes in the water at its Ohio River intake site. Still, the company installed a secondary intake on the Guyandotte River in case an alternate source was needed. The subsidiary, which serves more than half a million people, has also enhanced its treatment processes.

The Evansville Water and Sewer Utility in Indiana, which draws water from the Ohio River for its treatment plant, was also monitoring and testing the waterway.

“There is a slim chance that we will detect contamination from this spill at our site because our structure is around 700 river miles away from the spill,” said Ella Johnson-Watson, a spokeswoman for the utility.

Mr. DeWine, however, said that if he lived in the affected area, he would drink bottled water.

Just after the derailment, 1,500 to 2,000 residents in East Palestine were told to evacuate the area. Schools were closed for the week, along with some roads. Norfolk Southern said it had donated $25,000 to help the American Red Cross set up shelters and deal with the influx of people.

On Feb. 6, Mr. DeWine extended the evacuation order to include anyone in a one-by-two-mile area surrounding East Palestine, including parts of Ohio and Pennsylvania.

There have been no reports of injuries or deaths from the derailment, but on social media and in news reports, some residents said that fish and frogs were dying in local streams and people have shared images of dead animals or said they smelled chemical odors around town. The arrest of a reporter during a news conference about the derailment led to online criticism of the law enforcement response.

Residents of the area have complained of headaches and feeling sick since the derailment. A federal lawsuit filed by two Pennsylvania residents is seeking to force Norfolk Southern to set up health monitoring for residents in both states, and to pay for related care for those in a 30-mile radius.

The Ohio Environmental Protection Agency is working on a two-stage cleanup, starting with the removal of materials from the site before moving to an assessment for a remediation plan, a spokesman said.