In 2014, the E.P.A. proposed adding the site to its National Priorities List, a designation that means the agency can use Superfund program money to clean up the site, provided the state in which the site is located agrees and supplies about 10 percent of the costs.

Inclusion on the list does not automatically mean that the agency will pursue cleanup, or that liability will be assigned to a party or company. But it also allows the state and the federal government to try to recoup costs from companies deemed to be responsible for the pollution, the indictment says.

In 2014, the E.P.A. notified Drummond, whose ABC Coke plant operates at the site, and four other companies that they could be responsible for cleaning costs if the site were added to the priorities list.

That year, the agency also started to consider a petition from a Birmingham-based environmental health advocacy group, Gasp, to expand the size of the site to include two other affected areas: the Inglenook neighborhood and Tarrant, a nearby city, the indictment said.

But Mr. Gilbert, who was with the law firm Balch & Bingham, and Drummond worked to prevent the priority listing of the site and the expansion of it, the indictment said. Their plan included paying Mr. Robinson and “advising residents of north Birmingham and public officials to oppose E.P.A.’s actions,” it said.

It also said Mr. Gilbert wrote letters related to opposition of the listing of the site, which Mr. Robinson signed on his official stationery and presented to state officials to get them to oppose the E.P.A. plan.

Mr. Gilbert also “wrote a joint resolution” urging the attorney general and state officials to “combat the E.P.A.’s overreach” that Mr. Robinson, then a member of the Alabama House of Representatives’ Rules Committee, voted to send to the House after it was adopted by the Senate, the indictment said. The act was passed in 2015.