The brother of a U.S. journalist murdered in Moscow nearly five years ago said on Monday he was dismayed at Russia's failure to solve the killing of fellow journalist Anna Politkovskaya.

A Moscow court on Thursday acquitted three men of involvement in the 2006 murder of investigative reporter Politkovskaya, a step her former colleagues said showed the inadequacy of the Russian judicial system.

The case bore striking similarities to the trial of two men accused of shooting U.S. citizen Paul Klebnikov, editor of the Russian edition of Forbes magazine in 2004. The two suspects were acquitted.

"We were dismayed by the outcome of the Politkovskaya trial," Michael Klebnikov, Paul's elder brother, said by telephone from New York.

"Our impression is that little has been learnt since the botched trial of Paul," he said. "The system is not broken but it is certainly not working."

"Our concern is that the bottom line is that the open season on journalists is continuing and that would be unacceptable in most countries," said Klebnikov. "The Russian government should take immediate action to show its concern."

The people behind the murders of Klebnikov and Politkovskaya have never been convicted, despite pressure from both the United States and the European Union for convictions.

Politkovskaya, a 48-year-old mother of two, published exposes of rights abuses in the Novaya Gazeta newspaper. Klebnikov, a 41-year-old father of three, reported on the dangerous underworld where crime and politics overlap.

Both wrote about high-level corruption in Russia's southern region of Chechnya and ethnic Chechens were accused by Russian prosecutors of killing both reporters.

"We are urging the government to redouble its commitment publicly to fully investigate both Paul's and the Politkovskaya case and to commit itself to catching, indicting and convicting the masterminds of both cases," Klebnikov said.

Russia is ranked as the world's third most dangerous place for reporters by the New York-based Committee to Protect Journalists (CPJ), which lists 49 journalists killed in Russia since 1992. Only Iraq and Algeria had more.