Smoke detectors allegedly sealed at the behest of employees who light up in the corridors of the government-owned SDF Building in Sector V on Wednesday allowed a fire to spread through the second floor as the alarm system and the extinguishers slept, fire brigade officials said.

All smoke detectors in the six-storey building that houses more than 100 offices were found sealed with blue plastic caps so that they wouldn’t trip during smoke breaks, an official said.

“There are ‘no-smoking’ signs across the floors of the building, but few seem to care going by the cigarette butts and matchsticks on window sills and corridors,” the official said.

Nobody was injured in the fire that was detected around 10.15am and contained with four fire engines.

Employees had scurried out through the emergency exit of the building. Many of them knocked on the doors of offices on the second floor to alert others to the fire before running out.

Kundan Kumar, who works in an office on the second floor, said a smaller fire had broken out a few months ago but the smoke detectors remained sealed.

“A section of employees smoke in the corridors every day. They insist that the smoke detectors remain covered up. We had complained to the estate management and security guards about this but they did nothing,” another employee said.

Fire services minister Sujit Bose said a probe had started. “Fire services teams have already collected samples. We will take steps if we find negligence.”

A security officer in the building said that pump operator Apurba Kumar Banerjee first spotted the fire.