About a mile and a half from a maximum-security Salvadoran prison on Thursday, Senator Chris Van Hollen’s caravan was brought to an abrupt halt by a military roadblock.

Mr. Van Hollen, a Maryland Democrat and a member of the Senate Foreign Relations Committee, had made the final stop of his visit to El Salvador, where he had hoped to meet with Kilmar Armando Abrego Garcia, a Maryland man whose unlawful deportation last month has become a flashpoint in the debate over U.S. immigration policy.

Mr. Van Hollen and Chris Newman, a lawyer representing Mr. Abrego Garcia’s family, were denied their request to visit the prison, known as CECOT, and forced to turn back. The refusal of entry came a day after El Salvador’s vice president rejected Mr. Van Hollen’s formal appeal for a meeting or even a phone call with Mr. Abrego Garcia.

“Our purpose today was very straightforward,” Mr. Van Hollen said in an interview on Thursday. “It was simply to be able to go see if Kilmar Abrego Garcia is doing OK. I mean, nobody has heard anything about his condition since he was illegally abducted from the United States. He is totally beyond reach.”

After being stopped by the Salvadoran military officials, Mr. Van Hollen described the encounter as a blockade intended to thwart his visit to the prison. Human rights advocates have documented overcrowding in El Salvador’s prisons and reports of torture.

“This was a very sort of simple humanitarian request,” Mr. Van Hollen said soon after the stop. “They said they were ordered not to allow us to proceed any further.”

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