A day after the Russian navy intercepted the Maltese-flagged freighter Arctic Sea about 300 miles off Cape Verde, Defense Minister Anatoly Serdyukov said hijackers had seized control of the ship in Swedish waters on July 24 and forced its Russian crew at gunpoint to sail toward Africa.

"This was an act of piracy," he told reporters.

The statement was the first official confirmation that the ship had been hijacked. But Russian authorities said nothing about why anyone would seize an aging vessel carrying timber and declined to address glaring inconsistencies in accounts of the incident.

No ship has been hijacked in the Baltic Sea in several centuries, according to Swedish officials, and some security and maritime analysts said the sophistication of the operation pointed to state involvement and secret cargo, possibly nuclear material.

Finnish officials have confirmed that firefighters took the unusual step of conducting radiation tests on the Arctic Sea before it left Pietarsaari, in western Finland, in late July. The results were negative.

The freighter, operated by a Finnish company with Russian management, was scheduled to deliver a shipment of timber valued at $1.8 million to the Algerian port of Bejaia on Aug. 4.

But on July 28, Swedish police received a report that masked, uniformed men identifying themselves as drug enforcement agents had boarded the ship, assaulted and tied up the 15 crew members, and searched the vessel for half a day before letting them go, said Linda Widmark, press secretary for the Swedish National Police Board. The suspects reportedly arrived and departed in a high-speed inflatable dinghy marked "Police."

Widmark said the crew contacted the shipping company by e-mail after the alleged incident July 24, and the company informed the Russian Embassy in Finland, which reported it to Swedish police through diplomatic channels. Working with the company, Swedish police collected written statements from crew members, as well as photos showing their injuries, she said.

There was no sign of trouble when the freighter communicated with British authorities as it passed through the Dover Strait on July 28. Everything seemed fine July 31, too, when Swedish police spoke by phone with a man identifying himself as the captain, Widmark said.

But then the authorities lost contact with the freighter, prompting a two-week international search. Russia requested help from NATO last week, and the alliance shared data from a tracking system it uses to monitor ship movements, a NATO spokesman said. Meanwhile, Finnish police said the shipping company had reported receiving a ransom demand.

Serdyukov identified the suspected hijackers only by nationality -- two Russians, two Estonians and four Latvians -- and said they were being questioned aboard the naval frigate that intercepted the Arctic Sea. Naval forces freed the crew and apprehended the suspects without firing a shot, he said.

He said the hijackers had ordered the crew members to shut off the ship's communications and navigation equipment. But it was unclear whether the hijackers had ever left the ship and, if not, why they would have allowed the crew to report the July 24 boarding and search.

Mikhail Voitenko, a maritime security consultant and journalist who has been helping relatives of the crew members, said that the official version was full of holes and that the crime was beyond the means of ordinary pirates. Only "commandos" could pull off a hijacking in one of the world's busiest shipping lanes, within cellphone range, he argued, adding, "The operation cost more than the cargo and ship combined."