Ted Malloch, a political consultant connected to President Trump’s election campaign, said he was questioned by the FBI and served with a subpoena to testify before special counsel Robert Mueller, who is investigating the 2016 race and Russia’s involvement.

Mr. Malloch said in a statement Thursday that he was detained by FBI agents after arriving in Boston on Tuesday on an international flight from London and subsequently grilled about matters including his work with Mr. Trump’s 2016 presidential campaign.

“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,” Mr. 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.

Federal agents also confiscated Mr. Malloch’s cellphone and said it would be analyzed for a “full assessment,” he said in the statement.

Mr. Malloch is scheduled to appear before the special counsel’s grand jury on April 13, the statement said.

The special counsel’s office declined to comment, according to multiple news reports.

Mr. Malloch, 65, advised the Trump campaign on economic policy issues shortly after the president announced his candidacy, Germany’s Der Spiegel reported previously. More recently, he was widely rumored in 2017 to be considered as Mr. Trump’s pick for ambassador to the European Union.

“What could they want from me — a policy wonk and philosophical defender of Trump? I am not an operative, have no Russia contacts, and — aside from appearing on air and in print often to defend and congratulate our President — have done nothing wrong,” Mr. Malloch said.

“I worked with Ted Malloch from the very start, and if anyone can give an honest inside account of what happened during the course of the election cycle, it would be Ted,” senior White House adviser Sam Clovis said previously. “Ted’s perspective from being ‘off shore’ provides great perspective in analyzing what happened to get America headed back to greatness.”

The U.S. has assessed that Russia interfered in the 2016 presidential race, and Mr. Mueller was appointed last month to investigate Russia’s involvement, as well as any alleged coordination with the Trump campaign and “any matters that arose or may arise direction from the investigation.”

Mr. Trump has denied any collusion between his campaign and Russia.

Mr. Stone, 65, served as an early adviser to the Trump campaign and currently acts as a contributor for the far-right Infowars website that first reportedMr. Malloch’s detainment. He has recently come under fire over his alleged ties to WikiLeaks, the secret-spilling site that published documents damaging to Democratic candidate Hillary Clinton during the 2016 campaign, after having previously boasted of being in contact with its publisher, Julian Assange.

“This is the new McCarthyism — facts and evidence have been replaced by wild and baseless charges and guilt by association. Neither Ted Malloch or I know anything about Russian collusion in the 2016 election,” Mr. Stone said.

Mr. Corsi, an author and the D.C. bureau chief for Infowars, called Mr. Mueller a “hack” and a “thug” in response to news of Mr. Malloch’s detainment.

“As the editor of WikiLeaks, Mr. Assange enjoys hearing people’s stories and is much better company than Mr. Mueller,” WikiLeaks said in a statement to The Washington Times over the weekend.