U.S.-born hedge fund manager Bill Browder won’t be able to “sleep peacefully,” a top Russian official said while promising to escalate efforts to arrest the prominent critic of Russian President Vladimir Putin.

"We won’t let [Browder] sleep peacefully," Russian Prosecutor General Yury Chaika said Friday, according to TASS, a state-run media outlet.

Browder has led an international campaign to impose sanctions on Russian human rights violators. The effort culminated in U.S. legislation named after Sergei Magnitsky, an attorney for Browder who died in government custody after accusing Russian officials of cooperating with mafia in a $230 million tax fraud scheme. Russia countered that Browder and Magnitsky are the criminals; efforts by Putin allies to reverse the sanctions helped fuel the controversy over Russian involvement in the 2016 presidential election.

“I think Russia and Russian citizens will make more powerful moves soon,” Chaika said.

Browder was detained in Spain at Russia’s request in May, but then released after Interpol told Spanish officials “not to honor” his arrest warrant. He has been lobbying internationally for Western powers to crack down on the Russian oligarchs who are allied with Putin and have major assets outside of Russia.

“If you go after Putin’s money via the biggest oligarchs, as the U.S. has just started to do, that will stop him,” Browder told Reuters in an interview published Friday. “That is a checkmate situation.”

The Browder-Magnitsky case is a significant part of the recent tensions between the U.S. and Russia. Putin’s government banned adoptions to the U.S. following the 2012 passage of the Magnitksy Act. It even surfaced in the controversy over Russian interference in the 2016 elections: Donald Trump Jr. agreed to meet with a Russian lawyer purporting to have negative information about Hillary Clinton, but his interlocutor “turned the conversation to adoption of Russian children and the Magnitsky Act, an American law that blacklists suspected Russian human rights abusers,” according to the New York Times.

“It became clear to me that this was the true agenda all along and that the claims of potentially helpful information were a pretext for the meeting,” Trump Jr. said after confirming the meeting, which he added produced no meaningful information.

Trump, Jr., accepted the meeting request after being told that “the Crown prosecutor of Russia” wanted to give the campaign information on Clinton. Russia doesn’t have a “crown prosecutor,” which led to some speculation that it was a reference instead to Prosecutor-General Chaika.

Putin, for his part, maintains that Browder and Magnitsky are the true criminals. “Underneath are the criminal activities of an entire gang led by one particular man, I believe Browder is his name, who lived in the Russian Federation for ten years as a tourist and conducted activities, which were on the verge of being illegal, by buying Russian company stock without any right to do so, not being a Russian resident, and by moving tens and hundreds of millions of dollars out of the country and hence avoiding any taxes not only here but in the United States as well,” Putin said at the Valdai Discussion Club in 2017.

The case also surfaced in congressional oversight of the FBI investigation into Russian election interference. Fusion GPS co-founder Glenn Simpson, who commissioned the private investigation that produced the infamous “Steele dossier” about Trump, also had a history investigating Putin’s allegations about Browder in a separate case.

“I was extremely sympathetic for what happened to Sergei Magnitsky ... It was only later from this other case that I began to be curious and skeptical about William Browder's activities and history in Russia,” Simpson told the Senate Judiciary Committee in August of 2017. “[H]e tends to omit things that aren't helpful to him.”

Browder, whom Simpson described as fleeing a subpoena in a Magnitsky-related case, accused the investigator of airing “old and false Russian government attacks” on the pair. “He then whitewashes his accused Russian money launderer clients who received money from the crime Sergei Magnitsky was killed over,” he tweeted after the transcript of Simpson’s testimony was released.

Magnitsky died, after nearly a year in police custody, just days before Russian authorities would have had to release him or file charges. He was charged posthumously and convicted of tax fraud in 2012. Another Russian lawyer “suffered severe head injuries” in 2017 after falling from his apartment building while reportedly trying to move a hot tub. The incident took place one day before he was scheduled to appear in court on behalf of Magnitsky’s family.

"Once in a generation, strange things happen; when they happen on a regular basis like they do in Russia, there is government complicity in this," Maryland Sen. Ben Cardin, the top Democrat on the Foreign Relations Committee, told the Washington Examiner at the time. "These are intentional actions."

Chaika accused “Western intelligence services - from both the US and the [United Kingdom]” of protecting Browder. “We are well aware of what kind of person we are dealing with,” he said Friday. “Browder is a criminal, that’s clear to everyone, and we’ve got the verdict which came into legal force in Russia.”