The Centers for Disease Control and Prevention said multiple investigators in East Palestine, Ohio, fell ill while studying possible health impacts from the train derailment last month.
Residents of the town and the surrounding area had been complaining of symptoms such as lethargy and headaches ever since a Norfolk Southern train derailed Feb. 3, releasing vinyl chloride, ethyl acrylate and isobutylene into the environment -- chemicals considered to be very toxic, possibly even carcinogenic with high exposure.
First reported by CNN and confirmed by ABC News, seven investigators from the CDC and the Agency for Toxic Substances and Disease Registry -- part of the U.S. Department of Health and Human Services -- started experiencing symptoms.
"On March 6, seven members of a 15-person CDC/ATSDR team conducting Assessment of Chemical Exposure (ACE) surveys of East Palestine residents reported symptoms, including sore throat, headache, coughing and nausea," the CDC told ABC News in a statement.
According to the federal health agency, these symptoms are consistent with what residents and other first responders have described in the door-to-door ACE surveys.
The CDC said the seven people immediately reported their symptoms to federal safety officers.
"Symptoms resolved for most team members later the same afternoon, and everyone resumed work on survey data collection within 24 hours. Impacted team members have not reported ongoing health effects," the statement read.
It's unclear if the investigators' symptoms came from the toxic chemicals that had been released into the environment, but it comes after government officials and Norfolk Southern representatives repeatedly guaranteed the air and drinking water was safe.
Earlier this month, the U.S. Environmental Protection Agency said it would require Norfolk Southern to test directly for dioxins, a group of toxic chemical compounds.
The burning of vinyl chloride can release dioxins into the air. Dioxins are known as persistent organic pollutants, meaning once they are released into the environment, they take a long time to break down, according to the EPA.
If tests show that dioxins are found at a level that poses a risk to human health or the environment, the EPA will demand the company clean up the area.
The news of investigators falling ill comes the same day the Department of Justice filed a lawsuit against Norfolk Southern seeking injunctive relief and monetary penalties due to the derailment, including a nearly $65,000 fine for every day it violated clean water laws, court records show.