Environment protection officers say they are frustrated by the frequency of flaring at the Mossmorran chemical plant after receiving more than 380 complaints about the latest incident.
Unplanned flaring at the Fife plant began at 03:30 on Sunday.
Plant operator ExxonMobil has now indicated to environmental officers that this is due to a compressor fault.
It said it was planning to restart the equipment and flaring was expected to continue overnight.
The Scottish Environment Protection Agency (Sepa) said it was awaiting a full detailed report on the cause of the flaring but said it was investigating whether there had been a breach of permit conditions.
Monitors continue to capture data and officers are recording the impact on local communities.
Chris Dailly, head of environmental performance at Sepa, said: "Whilst its again important to stress that limited, controlled flaring is an authorised and important safety feature of industrial sites, we've heard clearly from over 380 reports to Sepa of the impact this further flaring event is having on local communities.
"Having referred ExxonMobil Chemical Limited to the Crown Office and Procurator Fiscal Service (COPFS) for consideration of prosecution related to Easter 2019 flaring, we remain frustrated by the frequency of flaring and the flow of information from the operator.
"We are investigating whether there has been a breach of permit conditions which would inform our next steps in line with our published Enforcement Policy."
He added: "We are clear on our expectations of the operator, including requiring the installation of noise reducing flare tips followed by the installation of ground flares.
"Moreover, the operator must take steps to make flaring the exception rather than routine and if they will not, Sepa will consider further options to ensure they do so."
He urged members of the public to report problems to Sepa.
ExxonMobil tweeted: "As our team undertakes the safe re-start of our compressor, you may see some fluctuations in our elevated flare tonight.
"We will continue to minimise the size of the flare wherever possible and apologise again for any frustration caused by this work."
The flaring is a safety measure at the plant but local people say it causes noise and light disturbance.
In May, Sepa confirmed it was seeking a prosecution following six days of flaring at the chemical works.