Sen. Chuck Grassley (R-IA) sent a letter to FBI Director Christopher Wray on Wednesday asking if the bureau ever warned the Trump campaign about ties between Russian officials and aides to the campaign.

In the letter, which Grassley released on Thursday, he noted that the Russia probe began in July 2016 and that recent reports show that former Trump campaign aide Paul Manafort was under investigation by federal investigators before the election.

“This raises the question of whether the FBI ever alerted Mr. Trump to the FBI’s counter-intelligence concerns regarding his campaign manager and others associated with the campaign — so that he could take defensive action to prevent the campaign from being infiltrated,” Grassley wrote in the letter.

The senator cited a June piece from Circa reporting that federal authorities warned Sen. John McCain’s (R-AZ) 2008 presidential campaign about ties between Paul Manafort and Russia. McCain’s campaign manager, Rick Davis, was Manafort’s business partner, and McCain met with Russian oligarch Oleg Deripaska during the campaign. Manafort reportedly offered Deripaska private briefings on Trump’s campaign in 2016.

However, McCain’s office said that he does not recall any such warnings from federal officials.

“Neither Senator McCain nor anyone on his staff recalls receiving such warnings from the intelligence community. Senator McCain had two interactions with Mr. Deripaska in 2006, and both were social occasions and entirely incidental,” McCain spokeswoman Julie Tarallo told CNN.

Tarallo said that any attempts to link McCain to Russian officials serve as a distraction from special counsel Robert Mueller’s probe.

“Facts are stubborn things, and the fact is no member of Congress has done more to push back on Russian aggression, human rights abuses, and corruption than Senator John McCain,” she said in the statement. “Any suggestion to the contrary is clearly intended to distract from the serious ongoing investigations into Russia’s interference in our election system.”

Grassley told the FBI that, given reports that the McCain campaign received a briefing on Manafort, he would like to know whether the Trump campaign received a similar warning about Manafort or other aides.

“If the FBI did provide a defensive briefing or similar warning to the campaign, then that would raise important questions about how the Trump campaign responded,” the senator 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 asked Wray to respond by Oct. 4.