Earlier in the day, Singh and his 92-year-old mother were found dead at their residence.
An official spokesman of the Chief Minister's Office said while Singh's throat had been slit his mother Gurcharan Kaur was suspected to have been strangled.
Expressing outrage, the journalists union also referred to murders of two more scribes earlier this month-- Gauri Lankesh in Karnataka on September 5, and Shantanu Bhowmick in Tripura on September 20.
Singh, who was in his sixties, was a former News Editor of the Indian Express, The Tribune and The Times of India in Chandigarh.
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"We urge upon the Chief Minister of Punjab to immediately take steps to track down the killers of K J Singh and his mother to allay the fears of journalists community," the union said.
It also demanded enactment of a law at the national level for safety of the journalists.
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