Atlanta-based Norfolk Southern could be held liable for the cleanup of a train derailment in Ohio that released chemicals into the air, soil and water and prompted evacuations, according to the U.S. Environmental Protection Agency.

The EPA sent a notice of potential liability to Norfolk Southern’s deputy general counsel on Friday, outlining the agency’s cleanup actions and the potential to hold the railroad accountable for the cleanup or associated costs. The federal notice comes as some nearby residents have sued the railroad and others have raised fears about long-term environmental contamination.

About 50 cars of a Norfolk Southern train derailed Feb. 3 in East Palestine, Ohio, between Youngstown and Pittsburg, Pennsylvania. Among the derailed cars were five hazardous materials cars carrying vinyl chloride, which is used to make plastics. Hundreds of residents evacuated the area.

To avoid a possible explosion, Norfolk Southern conducted a controlled burn of vinyl chloride three days after the derailment, creating a large plume of smoke that discharged toxic fumes into the air, according to Ohio Gov. Mike DeWine’s office.

By Feb. 8, air quality samples showed readings “below safety screening levels for contaminants of concern,” and health officials told residents they could safely return. DeWine’s office said those who remained uncomfortable returning home could ask the railroad for help with hotel expenses.

Norfolk Southern said it confirmed to EPA “that we have and will continue to perform or finance environmental monitoring and remediation.”

Norfolk Southern hired a contractor to take air quality samples for residents who want readings in their homes and testing of water from private wells. Last week DeWine’s office said those with private water wells were encouraged to use bottled water supplied by Norfolk Southern in the interim.

In its letter to Norfolk Southern, EPA said rail cars and tankers with vinyl chloride as well as butyl acrylate, ethylhexyl acrylate, isobutylene and ethylene glycol monobutyl ether were seen derailed, breached or on fire.

The agency also said in the letter that it observed materials from the derailment in the Ohio River and nearby creeks, and entering storm drains. EPA also said it observed areas of contaminated soil and free liquids “potentially covered and/or filled during reconstruction of the rail line including portions of the trench/burn pit that was used for the open burn off of vinyl chloride.”

Pittsburgh, Pennsylvania, television station WPXI-TV reported that a business owner and residents filed a federal lawsuit against Norfolk Southern, seeking class action status and seeking damages expected to exceed $5 million for people affected by the derailment.

Water service provider West Virginia American Water said Sunday it was monitoring water quality along the Ohio River after the derailment, and as a precaution was installing a secondary intake on the nearby Guyandotte River.

The city manager of Cincinnati wrote in a memo to the mayor and city council that “low levels of butyl acrylate have been detected in samples of the Ohio River downstream of the incident, but currently far upstream of Cincinnati.” Water services provider Greater Cincinnati Water Works is working with other agencies to collect samples and perform analyses of any potential threat to drinking water, according to the memo.

Norfolk Southern issued a statement shortly after the derailment saying it set up a family assistance center in East Palestine and made a $25,000 donation to the American Red Cross and their temporary community shelters.

The small size of the donation prompted criticism over the weekend from some people on Twitter, who said 5,000 people live in the area and cited Norfolk Southern’s market capitalization value of roughly $55 billion.

On Monday, Norfolk Southern said it has distributed more than $1 million to families to cover costs for the evacuation, and that its family assistance center has helped more than 700 families and some businesses.

Norfolk Southern said it has donated $220,000 to the East Palestine Fire Department for air packs, provided air purifiers for more than 100 residents, funded cleaning and air monitoring at East Palestine elementary and high schools, and is developing a charitable fund for the community. “This goes well beyond an initial contribution of $25,000 to the Ohio Red Cross,” Norfolk Southern said.

Norfolk Southern said the in-home air tests it has completed have “not shown any detections of substances related to the derailment,” and that water sample results will be available in the next week. The company said it is also sampling soil being excavated from the site of the derailment.

Norfolk Southern’s stock price has declined since the derailment, from $254.84 on Feb. 2. It was trading at about $239 Monday afternoon.