Zaur Dadayev, charged with involvement in the murder of Russian opposition figure Boris Nemtsov, looks out from a defendants' cage inside a court building in Moscow, March 8, 2015 REUTERS/Tatyana Makeyeva MOSCOW (Reuters) - A Russian judge on Sunday said a man detained on suspicion of killing opposition figure Boris Nemtsov had admitted to investigators that he was involved in the killing.

The judge at Moscow's Basmanny court ordered that Zaur Dadayev be held in custody until April 28, a Reuters reporter in the courtroom said.

Dadayev is one of five suspects who's being detained in the shooting death of Nemtsov, the Associated Press reported. Russia's Federal Security Service director Alexander Bortnikov didn't give details about how the five men were involved or how they were detained, according to the AP.

Nemtsov, a leading critic of Vladimir Putin, was reportedly planning to reveal Russian ties to the conflict in Urkaine when he was shot dead.

Many of Nemtsov's supporters have assumed Putin was behind his murder, but Russia's top investigative body doesn't seem to be entertaining that possibility. That investigator previously said it was considering the possibility that Nemtsov was killed to make Putin look bad, or that he was killed by Islamic militants.

Nemtsov, a 55-year-old former physicist, was a deputy prime minister and regional governor in the '90s who advocated for free market reforms.

(Reuters reporting by Katya Golubkova; Writing by Christian Lowe; Editing by Jeremy Gaunt)