Mohammad Ikhlaq and his 22-year-old son were dragged from their house by around 100 villagers in Dadri in Uttar Pradesh and beaten with bricks. Ikhlaq's son is critical.
His older son Mohammad Sartaj, 24, is an engineer with the Air Force.
Half an hour before the attack, an announcement was allegedly made at a temple nearby that a calf had been slaughtered and its carcass had been found near a transformer.
No names were taken in the announcement, but there are only two Muslim families in the area and the other family was out.
At around 10.30 pm, a large mob allegedly descended on Ikhlaq's house, wrecked everything and even attacked his wife and 70-year-old mother.
"My husband's head was smashed in," said Ikhlaq's wife.
The men were still being beaten when the police arrived. Six young men were arrested from the spot. The police say they were all drunk.
Ikhlaq's daughter said they only had mutton in their fridge, a gift from relatives on Eid. The police say they have taken the meat and sent it for forensic testing.
Killing cows is illegal in Uttar Pradesh.
The police are investigating who started the rumours in a part of western Uttar Pradesh that is prone to trouble at the slightest provocation.
Chief Minister Akhilesh Yadav said those involved in the killing "would not be spared."
News of the arrests led to tension in the area as villagers clashed with the police, allegedly forcing them to fire in the air. Villagers have claimed that a boy was injured in the firing.
As various accounts swirled on social media, the district administration warned trouble-makers. "If a digital war against communal harmony is initiated then the guilty will be punished. But our first concern is to stop any such thing and we are working on it," said NP Singh, the district magistrate.
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