Forensic police said the man, aged in his thirties, has died for at least a week before.
Suicide was first believed to be the cause.
Police later identified the tourist as Mr Stig Johan Kristian Hammarsten, 30.
He was found hanged to a beam at an bandoned construction site by a photographer who had sneaked past the guard into the building which is famous for its spectacular sunrise view over the Chao Phraya River in Bangkok.
But it took a while before people believed the photographer’s story.
The photographer, Natthawat, also chef at a hotelrestaurant, rang the traffic radio station, Jor Sor 100, which alerted Yannawa police to inspect the scene.
He said he found the body in the 49-storey Sathorn Unique Tower building, on Chaoroenkrung Soi 51 off Charoenkrung Road while going up there to take photos as part of his hobby of photographing bird-eye’s views of the capital.
The building was deserted for over a decade during the major financial crisis.
He told the station he paid 100 baht as fee to the building guard while foreigners would be charged 300 baht.
However, he earlier posted on Pantip.com web board admitting that he slipped through the deck via back stairs without seeking guard permission.
In the post, he said he had been to the building several times for photographing because it is one of the best location to take panoramic view of the capital by the river.
He told JS 100 station that after taking photos on the roof-deck of the 49-storey building at about 11 am on Friday, he went down and smelt bad ordour coming out a bathroom on the 43rd floor. When he came in, he found the body hanging on a beam.
Shocked by the grisly encounter, he tried in vain to contact police. But he said he told what he found to concerned authorities which included 191, 199, 193 and even charity foundations but none of them paid much attention to his story.
He said he also walked up to a traffic police before boarding a sky train back home.
The officer just recommended what he should do but said he could not do much because it was not his area of responsibility.
He then returned home and felt very uncomfortably about what he found and whether any authority would trust him and check the building.
In the post on the leading website, he said he felt as if something was trying to push him to go back to the place which he said he didn’t know what it was.
He said he couldn’t sleep and therefore decided to post on the Pantip web board telling the whole story.
At first many posters didn’t trust what he posted saying he was making a story. But some recommended him to call JS 100.
The station later alerted Por Teck Tueng Foundation rescue workers who went to the building with the police at 7 p.m.
However they didn’t go up waiting for him to arrive and guide them to the place.
He said it was until 10.30 pm Friday night that he arrived and led them up and found the body.
A wooden stair, a rice box and an empty can of beer were found near his body. A black wallet containing a driving licence issued by Swedish authorities and two credit cards was found in his jeans.
Immigration documents showed he arrived at Suvarnabhumi airport on November 10 and stayed at a guest house on Khao Sarn road.
But he went missing from his rented room since November 20 and the guest house has open the room for other customer to rent.
Yannawa police superintendent Pol Lt-Col Sanchai Matkamcan said preliminary examination indicated the Swedish backpacker committed suicide.
Police said they would contact relatives of the Swedish victim whose name was on the driving licence. His body was sent to Police General Hospital for autopsy.