At a private meeting in September, congressional aides asked Rebeckah Adcock, a top official at the Department of Agriculture, to reveal the identities of the people serving on the deregulation team she leads at the agency.

Teams like Ms. Adcock’s, created under an executive order by President Trump, had been taking heat from Democratic lawmakers over their secrecy. What little was publicly known suggested that some of the groups’ members had deep ties to the industries being regulated.

Ms. Adcock, a former pesticide industry executive, brushed off the request, according to House aides familiar with the exchange, who asked for anonymity because they were not authorized to comment publicly. Making the names public, they recalled her saying, would trigger a deluge of lobbyists.

In fact, interviews and visitor logs at the Agriculture Department showed that Ms. Adcock had already been meeting with lobbyists, including those from her former employer, the pesticide industry’s main trade group, CropLife America, and its members. CropLife pushes the agenda of pesticide makers in Washington, including easing rules related to safety standards and clean water.