A Kremlin critic was arrested in Spain under a Russian warrant. Officials can't explain why.

A leading Putin critic who has lobbied for tough anti-Kremlin “Magnitsky Act” sanctions around the world was briefly arrested under a Russian warrant in Spain Wednesday. But it’s not clear exactly why.

British businessman Bill Browder was released about an hour after his arrest in Madrid, where he was due to give evidence to a Spanish prosecutor regarding a corruption case involving Russian officials. He’s been a driving force behind sanctions against Russian oligarchs in the U.S. and Europe.

Tweeting from outside a Madrid police station shortly after his release, Browder said it was “the 6th time that Russia has abused Interpol in my case.” A Russian court sentenced Browder to nine years jail in absentia in December after finding him guilty of deliberate bankruptcy and tax evasion, and Moscow has repeatedly sought his arrest through Interpol.

Browder has always said that the charges against him are politically motivated, and Interpol has agreed, refusing repeated Russian requests for his arrest as political in nature.

There were conflicting accounts for the reasons for Browder’s arrest and swift release this time.

A Spanish police spokesman said that Browder had been mistakenly arrested as a result of an expired arrest warrant that had not been deleted from the system, and that he was released as soon as officers realized the international warrant was no longer valid.

Interpol said that there had never been a “Red Notice” issued for Browder, and that he had never been sought through Interpol.

Browder said on Twitter he had been arrested as a result of a live Russian request for his arrest through Interpol channels – and that Interpol’s general secretary had advised Spanish police not to honor the request.

“It was NOT an expired warrant, but a live one,” he tweeted. “Interpol is incapable of stopping Russian abuse of their systems.”

Browder, head of investment fund Hermitage Capital Management, was once the largest foreign investor in the Russian stock market before he was kicked out of Russia.

Sergei Magnitsky, a Russian lawyer who worked for his company, was arrested in 2008 after alleging Russian officials were implicated in a huge tax fraud. Magnitsky’s death a year later, while he was in jail awaiting trial, spurred Browder into a lobbying campaign that persuaded the U.S. Congress and European Parliament to pass so-called “Magnitsky Act” laws sanctioning Kremlin officials linked to the lawyer’s death.

Browder says that as a result of his lobbying, Russian President Vladimir Putin has developed a personal vendetta against him, repeatedly seeking his arrest through Interpol. In 2016, the Council of Europe criticized Moscow’s attempts to have Browder extradited through Interpol as “abuses” of the system.

Browder tweeted from Madrid airport Wednesday that he had cleared immigration and was looking forward to heading home to the U.K., while British Foreign Minister Boris Johnson said he welcomed his release.

“Moscow should concentrate on bringing those responsible for the murder of Magnitsky to justice,” he said in a tweet.

Cover image: Chief Executive Officer and co-founder of the investment fund Hermitage Capital Management Bill Browder attends the ‘Prospects for Russia after Putin’ debate in the Houses of Parliament, central London on Nov. 18, 2014. (LEON NEAL/AFP/Getty Images)