More credence should have been given to concerns about the North’s Renewable Heat Incentive (RHI) scheme, the head of the Northern Ireland Civil Service said.
Janette O'Hagan was selling a heat efficiency product in 2013 but found potential customers were not interested after they had signed up for the botched green energy scheme. She raised the alarm over potential misuse.
The overspending RHI led to the collapse of powersharing at Stormont 13 months ago after former deputy first minister Martin McGuinness resigned over the DUP's handling of the affair.
Civil service chief David Sterling said: "More credence should have been given to the concerns expressed by the concerned citizen."
Warnings were also issued by the regulator Ofgem.
The key decision-making meeting with former enterprise, trade and investment minister Arlene Foster was not minuted.
The RHI ended up massively overdrawn amid allegations of a cash-for-ash scandal.
Mr Sterling told a public inquiry at Stormont that once the RHI was up and running, there was a sense that it had developed a bit of momentum.
He said: "I think that the various warning signals that had occurred in different areas, we should have drawn a link between those, paused and said 'hold on, I am not sure this is right' - it did not happen.
"As the frequency of the warning signs increased during 2013 and into 2014, it should have been escalated some time during that period."
Lucrative measures to incentivise the burning of wood fuel were aimed at businesses and were used for purposes like heating chicken houses.
They ended up paying out more than it cost to fuel the heaters and led to a spike in subscribers to a scheme partly incentivised directly from Great Britain.
Mr Sterling highlighted a variety of weaknesses and wished he had been more "inquisitive".
He was permanent secretary at the Department of Enterprise, Trade and Investment - which established the RHI - with only three core members of staff directly involved day-to-day at one stage.
Mr Sterling said he wished he had had the foresight to direct more resources towards project management as the novel and flagship RHI policy was worked up.
He gave evidence to the RHI public inquiry at Stormont, and chairman Sir Patrick Coghlin queried why such a volatile and unpredictable scheme did not attract more of his personal interest.
Mr Sterling said: "I should perhaps have been more inquisitive, I should perhaps have stood back and said maybe I do need to look at this and become more closely involved.
"There were a range of controls and factors which were in place at the time which I would have taken comfort that the scheme was going to be managed well."