The Environmental Protection Agency banned the sale on Thursday of Imprelis, a weed killer introduced this year that landscapers link to thousands of tree deaths around the country.
DuPont, which held discussions with the E.P.A. on the herbicide, suspended sales of the product last week and announced plans for a refund program. The company already faces lawsuits from property owners who lost numerous trees after landscapers began applying Imprelis to lawns and golf courses this spring.
A spokesman for the E.P.A., Larry Jackson, said the agency acted because data provided by DuPont showed that at least three types of evergreens — balsam fir, Norway spruce and white pine trees — were susceptible to damage or death from Imprelis.
In a statement, the agency said it was investigating whether the widespread tree deaths resulted from misuse of the weed killer, inadequate warnings or directions on the product’s label, its persistence in soil and plant material or other factors.
Kate Childress, a spokeswoman for DuPont, said that the data mentioned by the E.P.A. indicated vulnerability only when the three tree species were exposed to “extreme conditions” in tests.
“We did them to understand the tolerance and sensitivity of these species under extreme conditions,” she said. Those tests were conducted before the product was approved by the E.P.A. last fall, she added.
Whether the product will return to the market after the agency’s review is unclear.