Senate Intelligence Committee Chairman Chuck Grassley is asking the FBI if it ever warned the Trump campaign about alleged Russian efforts to "infiltrate" the campaign.

In a Wednesday letter to FBI Director Christopher Wray, the Iowa Republican asked for details of any such defensive briefings, if the FBI ever did warn the Trump campaign, and how the Trump team responded. If no such briefings happened, Grassley said he wants to know why.

"If the FBI did provide a defensive briefing or similar warning to the [Trump] campaign, then that would raise important questions about how the Trump campaign responded," Grassley wrote. "On the other hand, if the FBI did not alert the campaign, then that would raise serious questions about what factors contributed to its decision and why it appears to have been handled differently in a very similar circumstance involving a previous campaign."

Grassley said Mr. Trump would need such an alert "so that he could take defensive action to prevent the campaign from being infiltrated."

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Federal officials wiretapped Manafort over concerns that he was communicating with Russian operatives who wanted to influence the U.S. election, a former U.S. official familiar with the intelligence confirmed to CBS News' Andres Triay. The Washington Post reported earlier this week that Manafort offered a Russian billionaire private briefings on the presidential campaign while he was the campaign manager.

In July, FBI agents raided his home, looking for proof that he set up offshore bank accounts, according to the New York Times. The New York Times reports federal officials have told Manafort he will likely be indicted.

Grassley gave the FBI until Oct. 4 to hand over a number of detailed records related to any such warnings or briefings, or lack thereof.

While Grassley's committee isn't specifically investigating Russian influence in the 2016 election — that's the job of the House and Senate intelligence committees — he is addressing related issues. The Senate Judiciary Committee subpoenaed Manafort in July, although it quickly dropped that subpoena.