The deceased were identified as Rana Nayek, 17, Shrabon Nayek, 19, Krishna Rabidas, 20, and Nipen Fulmali, 27 — all workers of Harinchhara Tea Garden under Finlay Tea Company.
Aminul Islam, officer-in-charge of Sreemangal Police Station, said five victims were rescued in critical condition and rushed to Moulvibazar 250-bed hospital where doctors declared four of them dead.
Another worker, Rabi Banarjee, 20, was initially given primary treatment and later shifted to Sylhet MAG Osmani Medical College Hospital for advanced care.
Dr Pronoy Kanti Das, superintendent of the Moulvibazar 250-bed hospital, said apparently the four died from toxic gas poisoning while working inside the septic tank, reports UNB.
Police said legal steps will be taken.