• They believe that three died after jumping in to rescue a worker who had been emptying the tank.
• The manure was being collected for use as fertiliser on the farm's fields.
Four Sikh men from Punjab have drowned in a slurry tank on a dairy farm near Pavia in northern Italy.
Investigators suspect that the four were overcome by carbon dioxide fumes from the cow manure.
They believe that three died after jumping in to rescue a worker who had been emptying the tank.
Two owned the farm and two were employees. All were Indian citizens. The two brothers Prem and Tarsem Singh registered their farm in 2017.
It is about 45km (28 miles) from Milan.
Italian media report that the Singhs' farm at Arena Po, producing milk and veal cattle, is one of the biggest in the Pavia region.
The victims were named as Prem and Tarsem Singh, aged 48 and 45 respectively, Arminder Singh (29) and Majinder Singh (28).
The manure was being collected for use as fertiliser on the farm's fields.
The victims' wives raised the alarm when the men failed to turn up for lunch on Thursday. They rushed to the scene and spotted one body in the sewage.
The wives then called in firefighters, who donned masks and emptied the tank to recover all the bodies.
Italy's public broadcaster Rai says the Arena Po tragedy brings to 486 the total of deaths in work accidents in Italy this year - the highest toll since 2016.
The new agriculture minister, Teresa Bellanova, tweeted condolences and said: "safety at work is an inalienable right and we must make every effort to ensure it is respected". She was a farm labourer herself as a teenager.