Attorney-general Solomon Berewa told journalists that the government had discovered this from documents found in Mr Sankoh's house.
Pro-government forces, meanwhile, have continued their advance against Mr Sankoh's Revolutionary United Front (RUF) rebels.
Mr Sankoh, whose forces are holding about 500 UN peacekeepers, is officially a member of the government, but has been missing since a peace protest outside his house on Monday.
Evidence
When demonstrators tried to enter the building, RUF fighters fired into the crowd, killing at least seven people; 12 others died in the fighting and confusion that followed.
Mr Sankoh, who had briefly peered out from his porch when the shooting started, disappeared shortly afterwards.
His house was ransacked by the enraged crowd, and government investigators have since been sifting through the documents found there.
The attorney-general said these provided evidence of the coup plan, and some of Mr Sankoh's associates had been providing more details of it.
"He was planning to stage a very violent and bloody coup this week, but for the grace of God that did not happen, Mr Berewa said.
He said they also showed that Mr Sankoh had been selling diamonds and buying weapons after the signing of last year's peace accords.
He said that the protest had upset the coup plan.
State funerals were held on Friday for the 19 people killed, whom the government has described as national martyrs.
Masiaka retaken
On the military front, pro-government forces entered the town of Masiaka 60km east of Freetown on Saturday, five days after it had been abandoned by UN forces who had run out of ammunition.
The bodies of RUF rebels were laid out by the roadside.
The BBC West Africa correspondent, Mark Doyle, says the troops appear to be a combination of the Sierra Leonean government armed forces and pro-government militias.
The militias have been fighting the rebels for many years.
Originally trained as hunters, they entered Masiaka waving magic potions that they say protect them from bullets.
High on drugs, they celebrated their advance by firing their weapons into the air with wild abandon.
Their advance follows logistical and co-ordination support from some 1,100 British paratroopers, most of whom are based in Freetown.
A British military spokesman said that some 800 marines have now arrived by ship to assist the paratroopers, if necessary.
Our correspondent says that the British have played a key role in co-ordinating the bewildering array of pro-government forces.
These include the UN troops, two sometimes mutually hostile parts of the Sierra Leonean army and the pro-government militias.