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By Bernd Debusmann Jr
BBC News, Washington
Authorities say seven US health investigators fell ill while probing the impact of the 3 February train derailment in East Palestine, Ohio.
According to the Centers for Disease Control and Prevention (CDC), the investigator's symptoms included nausea and headaches.
Locals in East Palestine have reported similar illnesses.
The train was carrying vinyl chloride and other potentially hazardous substances.
The CDC investigators formed part of a team that was conducting house-to-house interviews in the area of the derailment last month, according to authorities. They immediately reported their symptoms to federal authorities after they fell ill.
"Symptoms resolved for most team members later the same afternoon," the CDC said in a statement. "Everyone resumed work on survey data collection within 24 hours. Impacted team members have not reported ongoing health effects."
In the wake of the derailment, state and federal officials repeatedly sought to reassure East Palestine residents that local air and water supplies were safe. Residents, however, reported headaches, nausea, burning eyes and sore throats, sparking fears that their long-term health could be impacted.
Environmental officials have said that nearly 45,000 animals died as a result of the toxic train crash, although all were aquatic species.
One of the chemicals that the train was carrying, vinyl chloride, is a colourless, hazardous gas that is primarily used to make PVC plastic. It is also a known carcinogen and acute exposure is linked to dizziness, drowsiness and headaches. Prolonged exposure can cause liver damage and a rare form of liver cancer.
On Thursday, the US Department of Justice filed a lawsuit against the company that operated the train - Norfolk Southern - over environmental damage caused by the derailment.
The justice department said it plans to hold the company responsible for "unlawfully polluting the nation's waterways and to ensure it pays the full cost of the environmental cleanup," the lawsuit states.
Additionally, the lawsuit is seeking fines and a judgement that will hold the firm accountable for future costs associated with the environmental response to the derailment.
A separate lawsuit, filed by Ohio Attorney General Dave Yost last month, is seeking to recoup the state's costs and ensure that Norfolk Southern carries out long-term environmental monitoring.
Norfolk Southern has repeatedly apologised for the crash and has so far pledged $27.9m (£22.6m) to the community.
"I am deeply sorry for the impact this derailment has had on the people of East Palestine and surrounding communities," CEO Alan Shaw told a Senate committee earlier this month. "I am determined to make this right."