LONDON—The Moscow operation behind the now-infamous Russian-Trump Tower meeting in June 2016 included a direct attempt to enlist a foreign country’s law-enforcement official as a virtual double-agent, according to a court case in Switzerland.

One of Switzerland’s top investigators has been fired after allegations of bribery, violating secrecy laws, and “unauthorized clandestine behavior” in meeting with the very same Russian actors linked to the Trump Tower encounter.

Details of the explosive case have been published by investigative reporters for the Tribune de Genève and Tages-Anzeiger newspapers in Switzerland. The officer, identified only as Victor K., traveled to Moscow—against the expressed wishes of his superiors—where he spoke to Natalia Veselnitskaya, the lawyer who met with Donald Trump Jr., Paul Manafort, and Jared Kushner at Trump Tower.

The meeting was reportedly set up by Russian Deputy Attorney General Saak Albertovich Karapetyan—from the same rogue department that was apparently responsible for offering intel on Hillary Clinton to be shared at the Trump Tower meeting and the Kremlin’s further plots to influence U.S. politics.

The reports, which are based on Swiss court papers, describe how K. was lured to Moscow during a call from Karapetyan before Christmas 2016. He was told not to go by his boss, ostensibly because he was working too much overtime, but he made the trip anyway, using his diplomatic passport to fly to the Russian capital. There, he was put up in a luxury hotel and asked to attend an unexpected meeting with Veselnitskaya.

In the case against K.—who had been entrusted with investigating the Swiss financial arrangements of the Russian mafia and oligarchs for decades—it emerged that he had previously met Karapetyan in Geneva and Zurich, as well as Russia “without the knowledge of his superiors.”

According to the Tages-Anzeiger report, the Swiss Federal Administrative Court ruled this amounted to unacceptable “unauthorized clandestine behavior” that brought the integrity of the Federal Criminal Police into question. “In particular, it gives the impression that they do not have their employees under control and thus creates an unpredictable security risk in their delicate field.”

The interest of Veselnitskaya and the Russian prosecutor general’s office is likely to be linked back to a $230 million tax fraud that was uncovered by Sergei Magnitsky, a lawyer who was working for Bill Browder, whose Hermitage Capital had major investments in Russia. After discovering the massive financial crime that could be linked back to the Russian government, Magnitsky was arrested, beaten, and allowed to die in a Russian jail cell.

A series of laws in Magnitsky’s name have been enacted all over the world, imposing sanctions on Russians accused of corruption or complicity in the death of Magnitsky and the subsequent coverup. President Barack Obama signed the U.S. Magnitsky Act in 2012.

A large portion of those stolen funds are believed to have made their way into the Swiss banking sector, sparking investigations by Swiss authorities, including its federal police and the Swiss prosecutor’s office, both of which worked closely with K.

Alexander Perepilichnyy, a Russian businessman living near London, had tipped off Magnitsky about the role played by Switzerland in the international scam. A few weeks before he was due to give evidence at a hearing in Lausanne, he died suddenly while he was out running.

British detectives initially concluded there was nothing suspicious about the sudden collapse of the 44-year-old whistleblower—despite his own fears that he would be assassinated. A later toxicology investigation found traces of the deadly Gelsemium elegans flower, which is a known weapon of Chinese and Russian contract killers.

The Swiss court investigating K. heard that Veselnitskaya had raised the question of the Magnitsky case with him during that Moscow meeting. Swiss accounts linked to the fraud are still frozen.

Donald Trump Jr. and others have confirmed that Veselnitskaya raised similar issues during the summit at Trump Tower in June 2016.