WASHINGTON (AP) — Transportation Secretary Pete Buttigieg sent a letter Sunday to the CEO of Norfolk Southern, warning that the freight rail company must "demonstrate unequivocal support for the people" of East Palestine, Ohio, and surrounding areas after a fiery train derailment led to the release of chemicals and residents expressing concerns about their health.

"Norfolk Southern must live up to its commitment to make residents whole — and must also live up to its obligation to do whatever it takes to stop putting communities such as East Palestine at risk," Buttigieg wrote. "This is the right time for Norfolk Southern to take a leadership position within the rail industry, shifting to a posture that focuses on supporting, not thwarting, efforts to raise the standard of U.S. rail safety regulation."

Ohio Gov. Mike DeWine said Friday that the chemicals that spilled into the Ohio River are no longer a risk, even as people in the community say they have constant headaches and irritated eyes. The state plans to open a medical clinic in the village of 4,700 to analyze their symptoms, despite repeated statements that air and water testing has shown no signs of contaminants.

Still, uncertainty persists about the consequences of a derailment that occurred roughly two weeks ago.

Peter DeCarlo, a professor of environmental health and engineering at Johns Hopkins University, told ABC News on Sunday that more testing is needed to determine which chemicals are present.

"We just don't have the information we need to understand what chemicals may be present," DeCarlo said. "We know it started as vinyl chloride, but as soon as you burn that all bets are off. You have a lot of chemical byproducts that can happen from a combustion process like that."

Asked if he would move back to East Palestine if he were already living there, DeCarlo said: "I have two little boys. I would not."

Norfolk Southern CEO Alan Shaw issued a statement on Saturday that he "returned to East Palestine today to meet with local leaders, first responders, and a group of Norfolk Southern employees who live in the area."

"In every conversation today, I shared how deeply sorry I am this happened to their home," Shaw said. The Atlanta-based company has created a website with updates, NSMakingItRight.com.

The transportation secretary's letter on Sunday came across as a stern warning to Norfolk Southern, saying: "It is imperative that your company be unambiguous and forthright in its commitment to take care of the residents — now and in the future."

Buttigieg said the National Transportation Safety Board is investigating the cause of the derailment and that the Federal Railroad Administration is also analyzing whether safety violations occurred and will hold Norfolk Southern accountable if violations did occur.

The Environmental Protection Agency has said the company must document the release of hazardous contaminants and outline cleanup actions.

But Buttigieg also said that Norfolk Southern and other rail companies "spent millions of dollars in the courts and lobbying members of Congress to oppose common-sense safety regulations, stopping some entirely and reducing the scope of others." He said the effort undermined rules on brake requirements and delayed the phase-in for more durable rail cars to transport hazardous material to 2029, instead of the "originally envisioned date of 2025."

The transportation secretary said the results of the investigation are not yet know, but "we do know that these steps that Norfolk Southern and its peers lobbied against were intended to improve rail safety and to help keep Americans safe."