The leaders of the Senate Judiciary Committee say that they have reached an agreement to subpoena Paul Manafort, former Trump campaign chairman, to testify before the committee if he won't appear voluntarily.

The chairman of the committee, Sen. Chuck Grassley, R-Iowa, cautioned that there were still things to be worked out saying, "there's been agreement in principle, but the details have not been filled in yet."

This comes after CBS News confirmed earlier this month that Manafort was wiretapped under a foreign intelligence warrant in connection with U.S. concerns that he was communicating with Russian operatives who wanted to influence the American election.

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The U.S. government listened in on Manafort's conversations during the presidential campaign and through the election -- though not constantly -- and its surveillance includes the period when Manafort was Mr. Trump's campaign chairman.

The FBI raided his home in July and The Washington Post reported in early August that the search warrant was wide-ranging and FBI agents left his home with various records.

CBS News' Catherine Reynolds contributed to this report.