Newly unsealed court documents show that an adviser to the United Arab Emirates now cooperating with the special counsel’s investigation was convicted of transporting child pornography 27 years ago, the latest twist in the ongoing probe into Russia’s interference in the 2016 election.

It is unclear if the 1991 felony conviction of Lebanese American businessman George Nader figures into his agreement to assist special counsel Robert S. Mueller III.

But the case — and other records showing he repeatedly came under scrutiny by law enforcement agencies for similar behavior — raises questions about access Nader had last year to top White House officials, including President Trump’s son-in-law, Jared Kushner.

Court filings unsealed Friday show that Nader was stopped trying to bring child-pornography videos into the United States in 1990. He received a reduced sentence after influential figures argued privately to the court that he was playing an instrumental role at the time, trying to free U.S. hostages in Lebanon.

The new documents deepen the mystery around Nader, a shadowy figure who has served as a middleman to broker deals in the Middle East for three decades. Details in the voluminous case filing show that Nader was able to minimize his legal jeopardy in 1991 by arguing he was a pivotal figure in an ongoing national security crisis.

Sandeep Savla, a lawyer for Nader, called the revelations about his conviction “nothing more than an orchestrated, disgusting scheme by those who are trying to intimidate Mr. Nader into silence.”

Nader plans to “continue to answer truthfully questions put to him by the special counsel,” he added.

Blackwater founder Erik Prince met with a Russian person close to President Vladimir Putin in Jan. 2017, according to U.S., European and Arab officials. (Sarah Parnass/The Washington Post)

Nader has been cooperating with prosecutors working for Mueller since he was stopped and questioned in January by FBI agents at Dulles International Airport after arriving from overseas, according to people familiar with the matter, who spoke on the condition of anonymity to discuss sensitive matters.

Nader could have knowledge of interactions between associates of Trump and Russian officials during the presidential transition. He helped organize a January 2017 meeting in the Seychelles between Erik Prince, the founder of the private security firm Blackwater, and UAE officials, as well as a Russian banker close to Russian President Vladi­mir Putin, The Washington Post previously reported.

Prince has said his meeting with Kirill Dmitriev, the head of a Russian government-controlled wealth fund, was an chance encounter that occurred because he happened to be meeting with UAE officials at a luxury hotel in the Indian Ocean nation.

But Nader has told investigators that the meeting was set up in advance, as a back channel between a Trump emissary and a Kremlin official, to allow for informal discussions of future relations between the two countries, people familiar with the investigation have said. Nader’s lawyers declined to comment.

Nader also visited the White House several times after the Seychelles encounter, meeting with both senior adviser Stephen K. Bannon and Kushner, according to people familiar with his visits.

It is unclear if White House officials were aware of his 1991 child-pornography conviction. A White House spokesman referred questions to White House attorney Ty Cobb, who declined to comment.

If Nader were involved in White House policy discussions or other significant matters, officials should have thoroughly inspected his background as well as foreign travel for signs of potential problems, said Joel Brenner, who served as the head of U.S. counterintelligence at the office of the director of national intelligence from 2006 to 2009.

“You’ve got to check his background in every country he’s been in,” Brenner said.

Born in Lebanon, Nader came to the United States as a teen and later founded Middle East Insight, a magazine dedicated to coverage of the region and a role that gave him entree to travel frequently, and interview world leaders and top U.S. politicians.

In the 1980s, he developed a reputation as a back-channel negotiator with access to top officials in Israel, Syria and Iran, as well as leaders of the Hezbollah movement, according to people familiar with his work. In the past few years, he worked as an adviser to top officials in the United Arab Emirates.

“He belongs in the category of intriguing personalities whose agendas were never quite clear, and who are used to a certain degree by various administrations, not so much to deliver messages, but to acquire information,” said Aaron David Miller, a specialist in the region who worked for Republican and Democratic administrations.

But amid his international work, Nader had repeatedly been investigated by law enforcement officials, the newly unsealed court filings show.

He was indicted by a federal grand jury in the District in 1985 on two federal counts of mailing and importing child pornography. Court documents show the charges were dismissed before trial after Nader’s lawyers successfully argued that authorities had illegally seized evidence in the case. Records show that Nader successfully became a U.S. citizen while awaiting trial in the case.

During two instances in 1988, Nader received sexually explicit material, featuring underage boys, sent to him via a post-office box in Cleveland, according to court filings. He was not charged, although his home was searched and prosecutors say child pornography was found in his toilet.

More recently, the Associated Press has reported that Nader was convicted of 10 cases of sexually abusing minors in Prague in May 2003 and sentenced to one year in prison. His expulsion from the country was also ordered. ­Nader’s lawyers declined to comment on the Prague case.

In the recently unsealed case, Nader was stopped in 1990 at Dulles Airport with two videotapes depicting teenage boys, engaged in sex acts, hidden in his luggage in a candy tin, court filings show.

He pleaded guilty to one count of transporting child pornography and served about six months in federal custody in a local facility on work release, records show.

Nader had powerful supporters who appealed to the court on his behalf, arguing that he was engaged in high-stakes negotiations to assist the U.S. government in freeing hostages in Lebanon.

Court documents include a July 1992 letter written to the prosecutor, urging leniency for Nader, by Guilford Glazer — a real estate developer who described himself in the letter as a friend of President Ronald Reagan and “an advisor and confidante to most of Israel’s Prime Ministers.”

Glazer, who died in 2014, said Nader had repeatedly met with Israeli intelligence officials and leaders of Hezbollah in the risky hostage-release efforts.

“The Israeli intelligence people have their own sources in Lebanon. They assure me that George’s contact with the heads of the Hezbollah is at this moment, the best possible opportunity for an exchange,’’ Glazer wrote, adding that such a deal could also include the release of six American hostages and six European ones.

Likewise, Nader’s lawyer argued to the court that top officials, including Reagan, held Nader in high regard. Jean Sutherland, the wife of an American hostage then held in Lebanon, felt so strongly about Nader’s work that she was prepared to fly from Beirut to Washington to be present for ­Nader’s sentencing, Nader’s lawyer argued. He noted that she wrote a letter to the court maintaining that “no one else at this time can do the work that Mr. Nader is doing in the region.”

Before pleading guilty, Nader was allowed to travel — including to Beirut and to Moscow, apparently as part of his work to free U.S. hostages. However, prosecutors in the case played down ­Nader’s role in Middle East diplomacy, arguing that he had been allowed to travel “on the outside chance that his alleged meetings” could help.

Nader was ultimately given consideration in his sentence because of what a federal judge termed his “extraordinary cooperation with the government in certain areas,” according to court documents.

In 1998, James A. Baker, secretary of state under President George H.W. Bush during Nader’s prosecution, publicly thanked Nader for his work during the episode in a speech he gave for Nader’s organization.

“Early on in the Bush administration, George Nader did some very discreet work for us of a humanitarian nature having to do with the then-ongoing crisis over the taking of hostages, American citizens as hostage. And George worked very closely and very discreetly with us on that,” Baker said.

Josh Dawsey, Karen DeYoung, Tom Hamburger, Shane Harris and Spencer S. Hsu contributed to this report.