(CNN) The FBI this week detained and questioned Ted Malloch -- who says he was an informal Trump campaign adviser in 2016 who was rumored last year to be a candidate for US ambassador to the European Union -- as part of special counsel Robert Mueller's investigation, Malloch said.

Malloch issued a written statement saying he was detained by the FBI on Tuesday after he arrived on an international flight to Boston, where federal investigators took his cell phone and questioned him about Republican operative Roger Stone and WikiLeaks. Malloch, who has authored a not-yet-published book accusing a so-called "deep state" within the US government of fabricating the opposition research dossier on President Donald Trump and Russia to try to destroy Trump, said he was served with a subpoena before he was released to appear before Mueller's grand jury in Washington, which he's expected to do next month.

"The questions got more detailed about my involvement in the Trump campaign (which was informal and unpaid); whom I communicated with; whom I knew and how well -- they had a long list of names," Malloch said. "They seemed to then focus more attention on Roger Stone (whom I have met a grand total of three times and with groups of people); Jerome Corsi, a journalist who edited a memoir I had written some years ago; and about WikiLeaks, which I knew nothing."

He said was asked specifically if he had visited the Ecuadorian Embassy in London, where WikiLeaks founder Julian Assange has been holed up for nearly six years. He had not, he said.

Malloch's statement shines another light into the workings of Mueller's probe into potential collusion between Trump's campaign team and Russia, which has also investigated potential financial crimes and other matters.