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Jakarta, Indonesia (CNN) -- Bad weather hampered recovery efforts Friday as rescue teams combed a forbidding slope of an Indonesian mountain where a Russian jetliner crashed on a demonstration flight this week.
Rescuers found 12 bodies early in the day, according to Vice Marshal Daryatmo, head of the National Search and Rescue Agency, who like many Indonesians uses only one name.
It will take at least two weeks to identify the victims through DNA tests, Indonesian authorities said.
All 45 on board the Sukhoi Superjet 100 are feared dead.
The Superjet 100, Russia's newest passenger plane, slammed into Mount Salak, a volcano south of Jakarta, after disappearing from radar screens Wednesday.
Most of the wreckage is on a steep slope about 6,000 feet high, making it difficult to reach.
The cause of the crash remained unclear. The Russian Investigative Committee said it has launched a criminal probe into possible safety violations.
"We can understand how the families are feeling right now, and we want to do this evacuation as fast as we can, but the problem is the crash site terrain is unreachable by parachute," Daryatmo said at a news conference Friday.
Indonesian President Susilo Bambang Yudhoyono announced a joint investigation Friday after a phone call with his Russia counterpart, Vladimir Putin.
"I welcome the offer from Russian President Putin because the goal is to investigate what could have caused the plane crash," Yudhoyono said.
The Russian Investigative Committee had said 48 people were on board the plane, including eight Russian crew members. But the Russian state-run news agency RIA Novosti said the number was 45, citing Sukhoi Civil Aviation President Vladimir Prisyazhnyuk as saying three of the people on the passenger list did not board the flight.
The plane was on a demonstration flight for Indonesian Ministry of Transportation officials and representatives of Indonesian airlines, the Russian Embassy in Jakarta said before the crash.
Indonesia's Sky Aviation signed a $380 million deal in 2011 to buy 12 Sukhoi Superjet 100s, and press reports said a number of Sky employees were on the plane that went down. Sukhoi employees are also among the missing.
It was the first crash of a Sukhoi Superjet 100, RIA Novosti said.
The plane was on its second demonstration flight Wednesday when it lost contact with air controllers at Jakarta's Halim Perdanakusuma Airport.
The Sukhoi jet arrived in Jakarta as part of a demonstration tour of six Asian countries. It had been to Myanmar, Pakistan and Kazakhstan, and was scheduled to visit Laos and Vietnam after Indonesia, RIA Novosti said.
Sukhoi manufactures military aircraft and is known especially for its fighter jets. Its civilian aircraft is narrow-bodied with a dual-class cabin that can transport 100 passengers over regional routes.
It flew its maiden flight in 2008 and has had encountered problems in the past.
In March, a Superjet 100 operated by Russia's Aeroflot Airlines was forced to abandon its flight to Astrakhan, Russia, and return to Moscow because of problems with the undercarriage, according to RIA Novosti.
A similar defect in another Aeroflot-operated Superjet 100 plane had to be fixed in Minsk in December. However, Russia's state-run United Aircraft Corp. said the defect did not affect passenger safety.
CNN's Kathy Quiano in Jakarta and Alla Eshchenko in Moscow contributed to this report.