The herbicide aminocyclopyrachlor was once a common weed killer used by the Oregon Department of Transportation and other public entities. Then, last week, Oregon became the first state in the U.S. to temporarily ban it from rights of way.

According to a report by Oregon Public Broadcasting, The Department of Agriculture is investigating potentially widespread tree damage because of the chemical.

“What we’re trying to get a handle on is, is this occurring in other areas? We are starting to hear about a situation in Eastern Washington,” said Rose Kachadoorian, pesticide program manager with ODA.

In Central Oregon, she said trees were poisoned in at least four locations. ODA has prohibited the use of products with aminocyclopyrachlor until April, and lasting regulation could be established.

Plans for this in-depth investigation come more than three years since a massive tree die-off in Deschutes National Forest was first linked to ODOT spraying. Nearly 1,500 Ponderosa pines near Sisters are standing dead or dying. Some are hundreds of years old. The Forest Service plans to log those trees, rather than risk a dead tree falling on Highway 20.

The right of way around the pines was sprayed with a herbicide called Perspective at least four years in a row until 2015. The tree roots may have absorbed the herbicide and then they died slowly. Kachadoorian said a pesticide investigator looked into the site in 2014.

“The damage at that point just wasn’t that noticeable, and as time went on the damage became more and more evident,” she said.

ODOT said it has already stopped spraying Perspective, a brand name made by Bayer. But it’s still approved for roadside weed control by federal regulators at the Environmental Protection Agency.

The label for Perspective has long included a warning about tree exposure, and a list of species vulnerable to small amounts of the herbicide, including Ponderosa pines.