It was a blowtorch that heated materials to a searing 800 deg C - twice as hot as it needed to be.
And in the end, it sparked a slow-burning fire that was undetected as a result of further fire safety lapses and human error.
That was the most likely cause of the blaze in the SingTel Internet-exchange room in October which resulted in a days-long stoppage of its services to many parts of Singapore, SingTel's board committee of inquiry concluded on Monday evening.
Comprising three independent members of the company's board, the committee also sharply criticised the telco's "outdated" cable maintenance system, which uses lead sealant to protect the exit points for cables.
This sealant needs to be heated and shrunk each time the cables need to be maintained.
On Oct 9, the SingTel employee who needed to heat a seal in SingTel's underground cable chamber at its Bukit Panjang exchange for cable diversion works borrowed a blowtorch from a contractor who was with him.
That torch was twice as hot as it needed to be, and it caused the sealant and plastic sheaths on the cables to overheat and smoulder, eventually catching fire.
All this went undetected because the employee - a veteran who had been with SingTel for 30 years - neglected to turn the smoke detectors back on when he went for lunch.
On top of that, the cable chamber was not equipped with an automatic fire suppression system, so the fire could not be put out when it was eventually discovered an hour later.
These were the conclusions of fire investigation experts, who ruled out four other possible causes: an electrical fire, a flammable gas explosion, a fire caused by a lit cigarette, and sabotage.
The committee's report slammed the use of lead sealants and open flames, calling it an unnecessary fire risk. It also called the use of lead "outdated" and a "health hazard".
The committee also engaged consultants to assess SingTel's network resiliency and public relations. The telco did much better on both these fronts, and Bell Labs praised its use of "innovative processes" such as multiple teams to splice cables in carrying out repairs.
SingTel chairman Simon Israel said the company's management accepted all of the committee's findings and recommendations.
The telco will switch - by the end of next year - to alternative sealing methods that do not require hot works and use flame-retardant PVC instead of polyethylene.
It also pledged to strengthen fire monitoring and work with the telecommunications industry to collaborate on disaster recovery, among other initiatives.
The worker concerned has been suspended with pay, and SingTel management will now decide on appropriate disciplinary action.
Meanwhile, the Infocomm Development Authority said its own investigations into the fire would wrap up in the next two months. The regulator sets and imposes fines for service lapses.
Get a copy of The Straits Times or go to straitstimes.com for more stories.