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 the concerns were dismissed as paranoia, another official said.

Other American officials who raised the list said that when they followed up they were told the Mexican officials never received it, that the list was too vague to act on or that junior officials misplaced it.

Eventually, Gen. Glen VanHerck, the head of U.S. Northern Command, aired the concerns in public testimony to the Senate in March 2022.

“The largest portion of G.R.U. members in the world is in Mexico right now,” said General VanHerck, referring to Russia’s main military intelligence agency. “Those are Russian intelligence personnel, and they keep an eye very closely on their opportunities to have influence on U.S. opportunities and access.”

Mr. López Obrador then brushed the general’s remarks aside.

By late 2022, Washington had grown so concerned over Russia’s spy buildup that Wendy Sherman, then deputy secretary of state, raised the issue with Mexico’s foreign minister, according to several people with knowledge of the meeting. (Ms. Sherman declined to comment.)

Marcelo Ebrard, the foreign minister at the time, tried to wave away the concerns, these people said, saying the Russians on Washington’s list were not “a problem.”

Ms. Sherman replied: “They are a problem. We know, we kicked many of them out of D.C. and now they are here.”

Mr. Ebrard promised to investigate, the people with knowledge of the meeting said. But every time American diplomats followed up, Foreign Ministry officials claimed they never received the list.

Mr. Ebrard declined to comment, and aides to Mr. López Obrador said he was not speaking to the news media.

In the following year, Washington’s concerns grew as hundreds of thousands of migrants, including Russians, gathered at the U.S.-Mexico border.

The Biden administration worried that Russia could plant spies among them, and dispatched Liz Sherwood Randall, the top homeland security adviser, to raise the issue with Mexican officials in several meetings, including with Mexico’s president, according to two people with knowledge of the talks. Ms. Sherwood-Randall declined to comment.

Mexico said it would keep an eye on Russian asylum seekers, according to those people, alerting U.S. officials to any suspicious behavior.