A spokesman for Mr. Grassley said Ms. Feinstein’s decision was “totally confounding.”

“Her action undermines the integrity of the committee’s oversight work and jeopardizes its ability to secure candid voluntary testimony,” the spokesman, Taylor Foy, said.

Mr. Simpson said he hired Mr. Steele in the late spring of 2016, and the former spy grew alarmed soon after getting to work. With Mr. Simpson’s blessing, he reached out to an old contact at the F.B.I. in early July.

“To me, it was like, you know, you’re driving to work and you see something happen, and you call 911,” Mr. Simpson said.

Current and former American and foreign officials with direct knowledge of the investigation say that the federal inquiry did not start with Mr. Steele’s dossier, early parts of which did not reach counterintelligence investigators at the F.B.I. until August, after the bureau’s inquiry had already begun. But the officials have said that the dossier added material and buttressed what American law enforcement and spy agencies were gleaning from other sources.

According to Mr. Simpson, it was not until late September, nearly two months after the F.B.I. investigation had begun, that the F.B.I. reached out to Mr. Steele. He then met with agents in Rome to brief them on his work.

At that meeting, Mr. Steele learned that his information was considered credible by the F.B.I. “because they had other intelligence that indicated the same thing and one of those pieces of intelligence was a human source from inside the Trump organization,” Mr. Simpson testified.