May 30, 2018, 9:56 AM UTC / Updated May 30, 2018, 10:14 AM UTC

LONDON — Bill Browder, an American-born investment fund manager and foe of Russian President Vladmir Putin, was briefly detained in Spain Wednesday on suspicion of having committed fraud.

Spanish national police later told NBC News that Browder had not been formally arrested and had been released after "checking a possible claim against his name."

The financier — who has led a campaign to punish Russian officials he blames for the 2009 death of lawyer Sergei Magnitsky — tweeted that he had been taken into custody in Madrid and was headed for a police station.

The London-based Browder, who is head of investment fund Hermitage Capital Management, also posted a photo which appeared to have been taken from the back of a police vehicle.

Bill Browder posted this photo on Twitter Wednesday with the message: "In the back of the Spanish police car going to the station." @Billbrowder / Twitter

Shortly afterwards, the Kremlin critic posted an image of a Spanish warrant which said he had been arrested on suspicion of fraud. The document did not reference Russia.

This is the arrest warrant pic.twitter.com/Cr1Sf4vS4a — Bill Browder (@Billbrowder) May 30, 2018

Browder's detention comes five months after a Russian court sentenced him to nine years in prison in absentia after finding him guilty of deliberate bankruptcy and tax evasion. The Russian Prosecutor General's office said that the tax evasion by Browder and co-defendant Ivan Cherkasov had caused some $58.76 million in damage to the Russian federal budget. Browder denies Russia's allegations against him.

Browder employed Russian lawyer Magnitsky, who was arrested in 2008 shortly after alleging that Russian officials were involved in large-scale tax fraud. Magnitsky died in a Moscow prison a year later.

Magnitsky had said he was denied medical care in an effort to persuade him to confess to tax evasion and give evidence against Browder. Putin has dismissed allegations of foul play against Magnitsky and said he died of heart failure.

In 2012, the United States passed a law known as the Magnitsky Act under which it has imposed visa bans and asset freezes on Russian officials linked to the lawyer’s death.

Browder has attempted to get several other countries to impose so-called Magnitsky sanctions against Russian individuals.

Earlier this month, Britain passed a so-called “Magnitsky Amendment” to its Sanctions and Anti-Money Laundering Bill, allowing the country to impose sanctions on people who commit gross human rights violations.

The amendment is not specifically aimed at Russians, but follows a low point in relations between Britain and Russia following a nerve agent attack in England on former Russian spy Sergei Skripal and his daughter Yulia in March. Britain blames Russia for the attack.

In March, former Russian spy Boris Karpichkov told NBC News that Browder was on a Kremlin "hit list" along with himself and Skripal, a former double agent.

Browder was once a supporter of Putin. He made millions investing in post-Soviet Russia and gave up his American citizenship in 1997.