FILE PHOTO: Senate Judiciary Committee Chairman Chuck Grassley (R-IA) meets with FBI Director nominee Christopher Wray on Capitol Hill in Washington, U.S., June 29, 2017. REUTERS/Joshua Roberts

WASHINGTON (Reuters) - The head of the U.S. Senate Judiciary Committee has asked the FBI whether it warned Donald Trump’s presidential campaign about alleged attempts by Russia to infiltrate the campaign.

Republican Senator Chuck Grassley’s office said on Thursday he wrote to Federal Bureau of Investigation Director Christopher Wray asking whether the agency provided “defensive briefings” to Trump’s team given its ongoing investigation of Paul Manafort, Trump’s campaign manager.

“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,” Grassley wrote in the letter dated Sept. 20.

If the FBI did not alert the campaign, Grassley said, 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.”

The senator said that according to press reports, U.S. intelligence had raised similar concerns with John McCain during the Republican senator’s 2008 presidential campaign.

Grassley, however, did not ask the FBI about the possibility that it had offered Trump or campaign officials such a briefing but the offer was declined.

After the 2016 election, two U.S. intelligence officers flew to New York to brief President-elect Trump on Russia, only to find when they arrived at Trump Tower that the briefing had been canceled, two senior U.S. officials said, speaking on the condition of anonymity.

Manafort became Trump’s campaign manager in June 2016 but was forced to resign two months later amid reports of his business relationship with the Kremlin-backed former Ukrainian leader, Viktor Yanukovich.

Trump denies his campaign colluded with Russia and dismisses investigations into allegations of Russian interference in the election as witch hunts. Russia denies meddling in the U.S. election.