never raised the issue of the bounty intelligence in his conversations with President Vladimir V. Putin of Russia. But after the C.I.A.’s assessment became public, senior military and diplomatic officials, including the secretary of state at the time, Mike Pompeo, warned their counterparts after all.

“If the Russians are offering money to kill Americans or, for that matter, other Westerners as well, there will be an enormous price to pay. That’s what I shared with Foreign Minister Lavrov,” Mr. Pompeo said in August during a trip to the Czech Republic. “I know our military has talked to their senior leaders, as well. We won’t brook that. We won’t tolerate that.”

Still, in testimony before Congress and in other remarks, senior Pentagon officials — caught between not wanting to aggravate the White House and not wanting to appear indifferent about the safety of troops — said they would be outraged if the C.I.A. assessment was correct, but also had yet to see definitive proof.

“It is not closed because we never close investigations that involve threats or potential threats against U.S. forces,” Gen. Kenneth F. McKenzie Jr., the head of the Pentagon’s Central Command, said late last year when asked about the status of the inquiry. “We’re looking at it very hard.”

Mr. Biden attacked Mr. Trump for failing to do anything about the C.I.A. assessment, portraying it as part of a strange pattern of deference he said Mr. Trump had shown toward Russia. Mr. Biden mentioning the matter in his speech accepting the Democratic nomination and brought it up in his first call as president with Mr. Putin.

While the sanctions imposed on Thursday were based on alleged Russian misdeeds other than the suspected bounties, the senior administration official said the diplomatic action about the available information “puts a burden on the Russian government to explain its actions, and take steps to address this disturbing pattern of behavior.”

The official added, “We cannot and will not accept the targeting of our personnel like this.”

Julian E. Barnes and Eric Schmitt contributed reporting.

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