MOSCOW, Russia (CNN) -- The hijackers who seized a Russian ship and its crew had demanded a $1.5 million ransom earlier this month, an official with the insurance agency told CNN on Tuesday.
The Arctic Sea was found by a Russian Navy vessel on Monday.
"The person spoke in English, apparently trying to change his voice to avoid being recognized, and introduced himself as a go-between," said Vladimir Dushin, vice president of Renaissance Insurance, the company that insured the vessel, the Arctic Sea.
A recording of the August 3 phone call was turned over to Russian special services, which launched an investigation, Dushin said.
The Arctic Sea was located Monday about 480 kilometers (300 miles) from Cape Verde, an island nation a few hundred miles from the coast of western Africa. It had been missing for more than two weeks. The 15-man crew was unharmed.
Russian authorities have opened a criminal investigation on charges of "kidnapping by an organized group of people," the Prosecutor General's Office said Tuesday in a statement.
Eight hijacking suspects -- from Estonia, Latvia and Russia -- were being interrogated, Russia's state-run RIA Novosti news agency reported. Crew members also were being questioned.
Dushin said he believes the man who said he represented the kidnappers got the insurance company's general phone number from the papers aboard the vessel, which are required under maritime law. The Arctic Sea has had a $4 million policy with Renaissance for several years, Dushin said.
The man on the phone demanded the $1.5 million ransom be paid within five days or they would kill the crew and sink the ship.
"If those demands are not met, or the insurance company passes this information to the police, (he said) the crew members will be killed one by one, and the ransom will be increased," Dushin said.
The Arctic Sea, which was sailing under a Maltese flag, had not been heard from since July 31. It was carrying 6,500 tons of timber from Finland to Algeria when it reported trouble on July 24 off Finland.
The crew had told authorities that eight to 12 people with guns and pistols boarded the vessel about 3 a.m. that day, masked and wearing uniforms with "police" written on them, the Malta Maritime Authority said.
It was scheduled to arrive in North Africa on August 4.
On Friday, the ship was reported in international waters north of Cape Verde. The news came from Portugal's state news agency, which quoted Cape Verde's defense director, Pedro Reis.
The U.S. military also had a report last week that the ship had been seen a few hundred miles from Cape Verde, two military sources told CNN, but the United States had no independent verification of those reports. The U.S. military was not involved in the search.
At the time, Russia's ambassador to Cape Verde denied that the Arctic Sea had been spotted near the island nation.
-- CNN's Maxim Tkachenko in Moscow, Russia contributed to this report