At least six workers suffocated to death and three were injured when they were overcome by toxic fumes while cleaning out a water pump, Civil Protection director Miguel Garcia told AFP.
"At present, six people have died, and the death toll could rise to nine," he said at the site of the 15 meter-deep well and pump in Hidalgo, in central Mexico.
Garcia said normal work procedures during cleaning operations call for closing down compartments inside the well, but that the National Water Commission had not given its clearance for the job.
"It seems they ignored the lack of clearance and, at first, four workers went down. Minutes later they reported back saying they felt ill from the fumes," he said.
Other workers descended into the well and also succumbed to the fumes, according to Garcia.
Medical reports indicate the injured workers "were in critical state because of their long exposure to the toxic gases that emanate from rotting organic matter," he added.