MOSCOW (AP) _ Radio correspondent Andrei Babitsky, who was detained by Russian troops in Chechnya and then allegedly turned over to Chechen rebels, is alive but his whereabouts are not known, a top security official told a news agency Monday.

Babitsky, a reporter for U.S.-funded Radio Liberty, was detained by the Russians and swapped Thursday to Chechen rebels for two Russian prisoners of war, officials said. The swap raised fears for his safety. The Russians said they no longer bore any responsibility for his security.

But the director of the Federal Security Service, Nikolai Patrushev, told the Interfax news agency Monday that Babitsky was alive. Patrushev said his agency did not know Babitsky's whereabouts and had played no role in the swap, which has been sharply criticized by Russian media and in the West.

In the comments picked up by Interfax, Patrushev did not say how he knew Babitsky was alive.

Sergei Yastrzhembsky, an aide to acting President Vladimir Putin, said that Patrushev was basing his claim on solid evidence. But Yastrzhembsky added that Russian authorities are still trying to locate Babitsky.

Yastrzhembsky, speaking at a news conference Monday, would not answer a question about how the Russians could have lost track of the rebels who took Babitsky, when the handover took place in an area of Chechnya that the Russians have long claimed to control.

The aide said Putin was told about the swap only after it had occurred.

Also Monday, the Russian prosecutor general's office said a summons had been issued for Babitsky to appear before investigators, and that he could face arrest if he doesn't show up. Sergei Prokopov, a spokesman at the office, said new information had been uncovered in Babitsky's case and that investigators wanted to question him.

Babitsky was originally detained because he didn't have proper accreditation to work in Chechnya, and some Russian officials later alleged he was aiding Chechen rebels.