Human rights groups have urged the Austrian government to bring to justice the killers of a Chechen dissident who was shot dead on a Vienna street.
Umar Israilov, 27, was killed by two gunmen on Tuesday, Austrian media say.
Amnesty International, Human Rights Watch and the Memorial Human Rights Centre said the victim had told police in Austria that he was being followed.
Mr Israilov, a former Chechen rebel, had accused the Russian-backed Chechen leader Ramzan Kadyrov of torturing him.
Mr Israilov, a former bodyguard for Mr Kadyrov who later fled, filed a complaint against the Chechen president with the European Court of Human Rights in 2006.
Austrian police arrested a suspect on Tuesday near St Poelten, 80km (50 miles) west of Vienna, in connection with the shooting, local media report.
Leadership under suspicion
The campaign groups said it appeared to be another politically-motivated killing of a critic of high-level Russian government officials.
"In light of the brutal retaliation inflicted on those who speak out on abuses in Chechnya, Israilov's actions were particularly courageous, and his killers and those behind them need to be promptly held to account," said Oleg Orlov, director of the Memorial Human Rights Centre.
"We're familiar with the case, and find Israilov's torture allegations entirely credible," said Rachel Denber of US-based Human Rights Watch.
Last September, Ruslan Yamadayev, a member of a prominent Chechen family who had fallen out with Ramzan Kadyrov, was shot dead in Moscow. Mr Kadyrov denied any involvement.
In a separate development, a Russian army officer jailed in 2003 for murdering a Chechen woman has been released before completing his 10-year sentence.
Yuri Budanov, first detained in early 2000, was granted early release for good behaviour on Thursday, Russia's Itar-Tass news agency reported. He had been held in Ulyanovsk, in central Russia.
He had confessed to strangling the 18-year-old woman in 2000, saying he had acted in a fit of rage while interrogating her, believing she was an enemy sniper.
Russian troops brought Chechnya under Moscow's control again in 1999 after a brief period of separatist rule.