King Birendra and eight other members of the royal family died in the shootings in the royal palace in Kathmandu.
The late king's uncle, Maheswar Kumar Singh, told the BBC that Prince Dipendra, King Birendra's eldest son, had opened fire on them with a machine-gun.
Another survivor, Rajiv Shahi, who is the son-in-law of the late king's brother Dhirendra, told journalists that the shooting spree lasted less than two minutes - after which Dipendra shot himself in the head.
Telling the truth
Mr Kumar Singh, 66, said he had refused to talk to the media before now - but he wanted the truth to get out.
He confirmed to BBC correspondent Daniel Lak initial reports based at the time on sources inside the palace that had requested anonymity.
These had said that the late Crown Prince Dipendra had used a sub machine-gun to massacre almost all of his immediate family.
Mr Kumar Singh said he had been standing with the late king and several others having a pre-dinner drink when the prince came in dressed in combat fatigues and carrying an assault rifle.
'Intoxicated'
He said the prince stood right next to him and fired a single shot into the ceiling, before letting loose a burst of automatic fire that hit King Birendra - others were also hit a few seconds later.
Queen Aishwarya and one of the royal princesses rushed into the room, then left screaming for help.
More shots were heard.
Mr Kumar Singh said he sought cover behind a sofa, along with the son of the current king, Gyanendra.
He says he heard several other machine-gun bursts then a few single shots that may have come from outside.
In his account, the other eyewitness, Mr Shahi, said that Dipendra appeared "very intoxicated".
Motive not known
He said Dipendra was darting in and out of the room where the family was gathered, shooting at them each time.
"He was staggering and falling now and then," he told a news conference in Kathmandu.
Mr Shahi, who is a doctor, says he rushed to help the injured King Birendra as he lay on the floor, trying to staunch his wounds.
The uncle, Mr Kumar Singh, said he had been talking to Prince Dipendra earlier in the evening and everything had seemed normal.
The killings have shocked ordinary Nepalese and left the country in a mood of deep uncertainty.
The new King Gyanendra has set up a commission of enquiry into the killings, which is expected to begin its work on Friday.
The BBC's Daniel Lak says the question now is if the eyewitness accounts will be accepted by ordinary people as the true version of events, or whether they will continue to believe conspiracy theories and rumours.
Local media have withheld coverage of eyewitness testimony, restricting themselves to re-broadcasting BBC material.
But one local website, nepalnews.com, has carried the details of Mr Shahi's news conference.