The missing AirAsia Indonesia flight QZ8501 is likely to be at the bottom of the sea, the head of Indonesia's search-and-rescue agency has said.
Bambang Soelistyo said the hypothesis was based on the co-ordinates of the plane when contact with it was lost.
The search for the Airbus A320-200, which disappeared with 162 people on board on Sunday on a flight to Singapore, has ended for a second day.
The search area will be widened on Tuesday.
Indonesia has asked for US help in searching for the plane, the US state department said, adding that it was considering the request.
The pilots had requested a course change because of bad weather but did not send any distress call before the plane disappeared from radar screens between Borneo and Sumatra.
"Based on the co-ordinates given to us and evaluation that the estimated crash position is in the sea, the hypothesis is the plane is at the bottom of the sea," Mr Soelistyo told a news conference in Jakarta.
At the scene: Sri Lestari, BBC News, over the Java Sea
I boarded a military aircraft at 06:00 at a base in Jakarta and was on board for 10 hours, seven of which were spent searching over the Java Sea near Belitung island.
It took us about two hours to reach the area where the authorities believe the plane was when it lost contact with air traffic control in Jakarta.
Our plane was flying very low - about 300-450m (1,000-1,500ft) above sea level. The weather was very clear and the waters relatively calm.
From the aircraft I could just see the water, a small island and a few fishing boats. The search team was trying to find wreckage or some signal from the missing plane. They were looking at the sea through small windows.
I did the same thing, checking the windows every 15 minutes. But I saw nothing.
At around 14:00 the search team found a slick of oil, but the authorities weren't sure where it had come from.
Announcing the end of the day's searching, Mr Soelistyo said that on Tuesday the search area would be widened to cover West Kalimantan, on the island of Borneo, and the southern parts of the waters off the coast of Belitung island.
Weather conditions on Monday were very good, he said, but helicopters involved in the search lacked the visual equipment for searching at night.
The search would continue to focus on oil slicks seen on Monday, Mr Soelistyo added, though it is not clear whether they were caused by the plane. Some ships were still searching for the plane, he added.
Indonesian Vice-President Jusuf Kalla said that 30 ships and 15 aircraft were taking part in the search and that any ships in the area and "even fishermen" were being asked to join in.
Flight QZ8501 had left Surabaya in eastern Java at 05:35 on Sunday (22:35 GMT Saturday) and was due to arrive in Singapore at 08:30.
The pilot radioed at 06:24 local time asking permission to climb to 38,000ft (11,000m) to avoid the dense storm clouds.
Indonesian officials said the request could not be immediately approved due to traffic but the plane disappeared from the radar screens before the pilots gave any further response.
It is unclear what happened next but one report suggests the plane may have tried to climb through the storm.
Former pilots say such a move could lead to reduced stability and possibly a fatal stall, as the plane is battered by ferocious cross winds and down draughts.
Modern aircraft are designed to withstand such pressures but experts say pilots can lose control in extreme circumstances.
The AirAsia Indonesia plane was delivered in 2008, has flown 13,600 times, completing 23,000 hours, and underwent its last maintenance in November.
The captain, Iriyanto, had more than 20,500 flight hours, almost 7,000 of them with AirAsia. The co-pilot is French national Remi Emmanuel Plesel.
The AirAsia group has previously had no fatal accidents involving its aircraft.
AirAsia boss Tony Fernandes has said the disappearance is his "worst nightmare".
There were 155 passengers on board, the company said in a statement
- There were 137 adults, 17 children and one infant
- Most were Indonesian but also one UK national, a Malaysian, a Singaporean and three South Koreans
- The BBC understands that the British national is Chi-Man Choi
- Two pilots and five crew were also on board - one French, the others Indonesian
This has been a difficult year for aviation in Asia - Malaysia's national carrier Malaysia Airlines has suffered two losses, flights MH370 and MH17.
Flight MH370 disappeared on a flight from Kuala Lumpur to Beijing in March with 239 passengers and crew. The wreckage, thought to be in the southern Indian Ocean, has still not been located.
MH17 was shot down over Ukraine in July, with all 298 on board killed.